Help me understand how to optimize bass on the Zu Definition Mk IV

I am a longtime owner of Zu Definition Mk1.5 speakers and recently also bought a pair of Definition Mk4's. I based this decision partly on the reviews / feedback posted here on Audiogon in various threads by members such as 213cobra, gsm18439, spiritofmusic and others. I'm really hoping some of you Mk4 owners can chime in and help me out here!

Basically, my issue is that I found the Mk1.5's put out incredibly deep and impactful bass, especially with some of the music I love, like R&B and reggae. And this was with the bass amplitude knob at 12 or 1 o'clock. However, after setting up my new Mk4's, I find the bass underwhelming and anemic, even after fiddling around with various settings and with the volume and PEQ cranked. I have them on spikes into bases on a hardwood floor, with about a 1.5" gap on the bottom. What am I missing here?? Based on the reviews I read, I was expected the bass to equal or exceed the lowly 1.5's. 

Thanks so much in advance for any helpful advice! 

So, room and location are the same as before?

First, the basics. Make sure all 4 woofers are working. :)

Next, placement. You might want to try the reverse speaker trick. Put the speaker in your normal listening location, and walk around the room until you hear the ideal bass. Then put the speaker there.

Temixoch, the bass on the Def4 is certainly a different beast than on my Def2. I'm assuming the 1.5 is similar to the 2, w four 10" subs rear firing.

The trick to getting it right is to drop the Low-Pass Filter to 38-42Hz. I'm at 38Hz, as is Phil. It's critical not to allow subs output to bleed into, and thus smear, the mids FRDs.

At 38-42Hz, have the Volume at 5-7.
I'm at 6.5, but this is really room dependent.

Other settings w me are
PEQ Gain 3.75
PEQ Freq 31
Phase 0° (both switch and dial)

For me, the 2s were just overwhelming, I could never get them right. Yes, bass on the 4s seems less powerful, but it's tighter and more tuneful.

Get yr 4s a little into the room, they start singing here 5' from front wall, best at 8'. From side walls, 3-4'.

Then set Low Pass Filter to 38Hz, Volume at 5/10. Repeat tracks w Volume going up in increments to 7/10.

Then start again w Low Pass at 39Hz, vary Volume incrementally 5-7/10. Then start again at 40Hz, incrementally Volume 5-7/10. And so on.

At some point hopefully you'll hit on the right balance of extension, bloom and speed.

Another consideration is the 4s are down firing. Thus, footers on the Zus and type of floor makes a difference, more than the 2s.

In my old apartment w solid concrete floors, stock spikes or Symposium Rollerblocks really worked. Here, on my 150 yr old suspended timber flr w plenty of flex, Arya Audio Revopods on Panzerholz are the ticket.

I'll ask Phil to chip in.

By any chance is your amplifier either bridged or "fully balanced"?

From one of the writeups provided for the Def IV at the Zu site:

Definition Mk.IV is designed around conventional or parallel mode amps, bridged amplifiers are not recommended (black speaker negative leg of bridged amps will see the virtual ground node of the internal bass amp).

The same caution would presumably apply to fully balanced amps as well as to bridged amps.


-- Al
Thanks for the great responses so far, very helpful!

Yes, these new MkIV's are in the same room and location as the previous speakers. Unfortunately, I am very limited where they can be placed, but the location worked very well before. They are only about 6" from the back wall and 2 feet from the side walls. Perhaps I was used to the walls reinforcing the bass so strongly with the 4 rear firing 10" woofers on the Mk1.5. Yes, the bass on the MkIV is tight and tuneful, but just pretty underwhelming so far in comparison (the down firing woofers are definitely functional). Hopefully I can get it dialed in, because it has left me with some buyer's regret, hoping for an upgrade, but feeling let down about the bottom end. For context, I am also a bass guitarist, so I do love some very deep low frequencies! 

Spiritofmusic, thank you for your very detailed post, appreciated! I will try some of those settings. I wish I could pull them out from the wall as you suggest, but it's just not practical at all for my space. Coincidentally, I also have a 150 yr old suspended wood floor, so I could play around with footers. I just put on the Black Diamond Racing cones that were on my last set of speakers. Have you found that the gap spacing to the floor is critical? In the manual, it suggests even 1/4" is sufficient, but at least one forum post I read said that bass didn't really begin to bloom until the speakers were 3" off the floor. 

As for my amplifiers, I don't think they are "bridged," but not 100% sure. They are Cary 300SE monoblocks, with a Cary SLP98 preamp in front, all run with unbalanced RCAs. One odd thing about the preamp is that it inverts phase, so you have to switch around the speakers cables, but I believe I have that sorted properly. 

Other thoughts are welcome!
For context, I am also a bass guitarist, so I do love some very deep low frequencies!

So when you get these the best you can then if you want a whole lot better get four cheap subs, place them near the walls spread asymmetrically around the room, and be amazed how deep and low and tuneful and focused the bass is with a distributed bass array.
@temicxoch As a SET amp the 300SE is neither balanced nor bridged, so you're fine in that respect.

Good luck.  Regards,

-- Al
Temixoch, good luck. There's no doubt that 4x 10" woofers firing 6" into a wall would produce a very pressurised bass, whereas 1x 12" firing down into the floor is a whole different concept.

My Revopods raise the Def4s 1.5" off the floor (I also situate the Zus on Panzerholz pieces).

FWIW, Jaco and Geddy sound pretty good here.
My Def4s are 6" from the back wall, and the floor is wood glued to reinforced concrete. Sean delivered the speakers, dialed in the bass amps, and the output curve is flat (see the owner's manual):

Low pass=45
PEQ Gain DB=3.0
PEQ frequency=35
Phase in degrees=0

Bass is tighter and more cohesive than either the 1.5s or 2s. You can always call Sean for suggestions.

GSM, I've just tried yr settings and really quite like the sound.
For the OP,

Are your Def 4s new or did you buy them used and they're simply new to you? There's a reason I'm asking. What are your room dimensions? Which 300B tube are you using in your SE amps?

I started with Def 1.5 back in early 2005. Then Def II, then (and still) Def 4. All three have been placed in exactly the same location. With the 4x10" back firing arrangement, I did have speakers placed close to the wall behind, but no corner loading. Under normal circumstances, the Def 4 12" sub should easily outperform the older 4x10" array, in bass depth, quality and amplitude. The old plate amp on the 4x10" wasn't as powerful as the sub amp in the Def 4. However, with 6" clearance to the back wall and 2' to the side, with the backfiring array you certainly had corner loading with the Def 1.5 that has gotten you accustomed to exaggerated bass. You might just have to wean yourself off some of that, because you are not going to get the same amplitude of corner loading from the downfiring sub in Def 4. Particularly with your 300B SET amps. You'd have corner reinforcement of rising THD bass.

On the other hand, Def 4 gives you control that laughs at the simple level knob on Def 1.5 / 2. So alot can be accomplished by getting the controls combination settings optimized. But first I need the answer to the questions whether your Def4 are new from Zu or used. Then I'll have more to suggest.

Again, thank you guys for the contributions. Please keep sending along any thoughts you have. So far they are exactly the kind of answers I was hoping for from current owners. Wish I could invite you all over for a drink and a listen!

So I played around for a few hours tonight and managed to improve the bass presentation on the speakers significantly. I've had to find my own settings, as the examples posted above just haven't done it for me, in my space, with my tastes. I would still say that these MkIV's in their current configuration here are outputting only maybe 70% of the bass quality I was getting from the Mk1.5's. But steps forward for sure. 

I've definitely had this realization.... I lived (very happily!) with my previous Zu's for 12 years here and had them dialled in just perfectly. I probably had somewhere around 5000 hours of listening bliss with them. Then I found these used MkIV's locally for a good price and decided to make the leap. Somehow I thought I could just slot them in just where the others were and figured it would be instant love. Now I get it... they are a different speaker, with a different solution for transducing bass frequencies, and I'm going to have to play around and get to know them in time... 

I was struck by reading this thread -- -- where 52tiger says he "was running out of options and really wasn't sure if the Def 4 was better than the def3," but after raising the height off the floor "the bass is tight and amazing, the downfiring design is so much better than the rear firing its no comparison." Clearly I have some experimenting to do. 

I'm having faith in the last line of that thread by dentdog: "With my Def 4s I worked them around quite a bit to optimize... For me it was well worth all the effort." 
Just saw your reply Phil. My MkIV's are new to me, bought used locally. I wasn't really looking to upgrade, but stumbled upon them and... well... you know how this hobby is. They look to be the first version (not rev B, if that makes a difference). My room dimensions are 13' wide by 22' deep, with the speakers on the short wall. They are 8 feet apart, tweeter to tweeter, with the listening position 8 to 9 feet from each tweeter. Almost a nearfield setup, which might be part of the issue in the way the bass is sounding in the sweet spot. Despite the size of the room, these positions can't really be changed, as the other half of the room is a music studio for playing and recording. 

My 300b tubes are Full Music 300b/n mesh plates. I've always loved them and they churned out mind-bending bass with my previous mk1.5s. 

However, you are right that I may need to wean myself off some of the exaggerated bass from the 1.5s. Being a bass player and a lover of some bass-heavy genres like reggae, those deep frequencies just floated my boat, even if it resulted at times in some smearing of the midrange. That said, I could barely turn the amplitude knobs past 12 o'clock without blowing my head off, while now I'm struggling to get enough bass out of the MkIV's. I do listen to a wide variety of music, so I'd like to get the balance correct!
The reason I asked about new vs. used is that in the early Def4, Sean set the internal amp's gain fairly low, such that in many rooms, including mine, the bass level had to be cranked to 8+, often 9, just to get in the realm. I pointed out to Sean that in a bass-shy room a user would not have enough sub amp gain headroom to properly load some rooms relative to the FRD efficiency. So he subsequently changed the internal gain setting on the internal amp so that the external pot yielded a far greater range of useful gain. I know all the Rev.B speakers were corrected, but some original series were too. I just don't know the serial # cut-off.

Call Sean with the serial numbers of your speakers, and he should be able to tell you if that's the case with your speakers, and he can send you a fix. If you have your sub gain cranked almost to 10, I believe that's your first order problem. In most environments, floor-to-plinth gap on Def 4 is practically immaterial. There are some situations where it is conceivable to matter but I have yet to actually encounter it. It doesn't hurt to experiment but I think you won't have to.

The TJ / Full Music Mesh Plate 300B (really a perforated plate) has three distinctive characteristics, compared to a more objective tube: a rich and euphonic top-end spray, classic coveted 300B lush midrange, and comparatively bloated bass that easily infects midrange transparency and articulation. It's a rich, creamy, decidedly not-objective sound. Now, all the Definitions have internal sub amps that derive their input signal from the bass character of the full range power amps. But the Def4 12" downfire is capable of much better bass articulation and is more dynamically disciplined, so you will have more to work with if you switch to one of the better solid plate 300B tubes, chief of which I suggest the KR 300B balloon glass. But if you need to spend less, one of the Sophia or TJ solid carbon plate tubes, or even the Shuguang Natural -T will be better.

IIRC Sean suggests new owners of Def4 start with the LPF set to 60 hz, which is right for almost nobody, but working down from there gives you an audible ramp into what works in your room. Then you can dial in amplitude and go after the PEQ settings to refine. But it seems highly-likely to me that you have an early pair of Def4 that made it into the field before Sean altered the internal amp gain to a more useful level.

Check and report back, or send my your serial #s and I'll call him tomorrow.


Phil... wow, amazing feedback! Yes, the mkIV's seem bass shy at reasonable volumes against the FRD efficiency (almost as if they were tuned brighter than my mk1.5s), but can clearly crank out deep frequencies when turned up. Just checked and the serial numbers here are 007 and 008, so I'd guess these are very early models. I'm working tomorrow and don't really know the Zu guys, so if you'd have time to call and ask Sean, I'd be very appreciative! 

I suppose I've liked the 300b mesh plate because I listen to so much music that isn't that well recorded and the rich and creamy character of these tubes hides some of that... 70s reggae made in the ghettos of Kingston just isn't audiophile grade. But I also have a strong collection of jazz, blues, R&B, and rock on vinyl as well, some of which are superb recordings. So for me it has been about finding a balance over the years of a system that's capable of being highly resolving, but also forgiving of poor recordings. It's a delicate dance. I'm certainly willing to try out some new tubes though ;)

Thanks again for all your help!
OK, David. I'll call Sean tomorrow and circle back. At 007/8, I am now certain this is your problem. I didn't get mine corrected because "9" gets me to the right spot, but if I moved while owning these I'd surely have to get the change myself. Standby.

Interesting and useful info from Phil. My Def4s are also a very early pair. Sean probably should update the downloadable owner's manual from the Zu website.

My Def 4s are nos.179 and 180. Where does this fit in the production run re sum amps' gain?
You're well past it, Marc.
Phil, you don't know the half of it LOL.

I'm trying GSM's suggestion of subs at 45Hz and 9/10 Level.

My default setting is 38Hz and 6.5/10.
Marc, while I was waiting for Phil to come back with info about the bass gain (keeping me in suspense haha!), I was reading some of the threads here and elsewhere about the Definitions. I noticed that you previous had Symposium roller blocks under yours, but now you are onto the Revopods. I happen to have a couple sets of roller blocks lying around that I used to have under my preamp and DAC.... how did you find them under the Zu's? I always figured spikes / cones would be better for speakers and roller blocks better with components that would be affected by micro-vibrations. What made you move on to the Revopods? (Though I might regret asking, since they are $$$!) David
Temicxoch, all this stuff is buyer beware, and critically, room dependent. My old space was a converted industrial loft with a bombproof solid concrete floor overlaid w timber, which meant that the option of Rollerblocks replacing spikes under the Zus was possible, and indeed these did have the edge on stock spikes.

Here, my floor is much more flexy, being an 1861 actual roof space. Even though I didn't realise it to start, the Rollerblocks were deleterious on this floor. I noticed existence of Revopods as I went to trial IsoAcoustics Gaias footers, tried both out, and the Revopods totally ruled. I can't say w any great confidence why this is the case, but for me the Revopods have really pushed my Definitions performance to the next level, incl the ability to get deeper, tighter and more extended subs output, which I know is your ongoing issue.

I'm not going to claim they are your solution, it's room setup and subs settings that needs to be yr current focus, but these footers could then be the icing on the cake.

For the record, the Revopods sit on Panzerholz slabs, themselves on Symposium Acoustics Svelte Shelves.
Thanks for the info! I do realize footers are less important than getting these speakers properly set up at this point. I was just curious to hear about your experience using roller blocks, since I have a couple sets kicking around. I never thought to try them under the speakers. It's a never ending quest, isn't it? :)
Give it a go, I certainly had success to start w RBs. Just take care on physically placing them under the Zus, they're obv not fixed.

Part of the joy of this hobby (or is that pain? LOL) is both incremental impvts, and occasional big leaps. 

For me, the Def4s have so positively responded to the system wide changes I've made, they've justified me not taking the easy option and simply upgraded to pricier, more complex spkrs.

I was able to speak with Sean Casey this morning. Your Def4s definitely are early enough to have to original lower-gain sub amps. You have s/n 007/008; I have s/n 009/010. You have three options, one of which requires you spend some money.

First, you can experiment with the Low Pass Filter frequency. It goes all the way to 110 Hz, so try frequencies above 60 Hz so you are well into overlap with the main drivers' bass response. I don't normally recommend this but it might work for you, and may require experimentation with the other controls to get it to sound right. You can run the level at 9, 9.5 or 10. No harm in trying.

Second, Sean says if you had Def2 and want some of the extra kick-drum slam that the 4x10" array delivered, you will get more of that out of Def4 using rubber feet instead of the stock spikes.

Third, you can simply remove your sub amp modules, return then to Zu for updating which takes about 4 days. The cost is $600/pr. They are not just making a gain change to the amp. The revision also includes Lundahl gain transformers embedded in the module. To arrange this, just call up Zu and talk to Sean or Gerritt. This will get you all the added gain you need, and you still may want to combine it with some measure of the first two options.

Thank so much, Phil. I really appreciate your help here!

Interestingly, I did try higher crossover frequencies (along with various settings of the parametric EQ) over the weekend and it was definitely an improvement. I'd still like some additional gain for more headroom, but further steps forward. With my ears not so focussed on the inadequate bass I was hearing previously, the upgraded drivers and Radian tweeters really began to shine. 

I wondered, too, if these speakers would benefit from a little less toe in (they are currently pointed directly at the listening position, only 9 feet away), compared to the mk1.5's, which were in the exact same spots. The spray of the Radians is glorious, but perhaps a little too prominent in the mix in this configuration. I haven't yet played with this, as moving these new Zu's is more than a one person job! 

I will make contact at Zu to upgrade the bass modules. I've come too far to stop now :)  

Once I get everything sorted, I'll close the loop and let you guys know how it all turns out...

Happy listening, 
Definition 4 setup uses less toe-in than Def 1.5 and Def 2 in every room where I have heard both. Particularly in a relatively near-field situation.  Both the Radian supertweeter and the FRD have broader dispersion than the respective drivers in the Def 1.5/2.

You can reduce toe-in with Def 4 without tearing a hole in the middle of the soundstaging. Get some help and adjust toe-in by quarter inch increments of rotation around the inside front foot or spike as the axis of rotation until you hear the soundstage lock in. You can start with no toe-in and incrementally add some or start with the toe-in that worked for Def 1.5s and incrementally reduce it. Trying from both reference points should be revealing because you want to end up in the same place regardless where you start. It helps to use recordings from before the multi-tracks / extensive multi-mics era (pre-1964) or modern recordings known to use simple mic'ing and minimal mult-tracking, to really nail it.

Also, by the way, David: To remove your sub amps, note that the Speakon connector there is flange surrounding the connector itself. Start by removing the screws from the Speakon flange. The Speakon connector and its wiring harness stay with the Definition. After you've done that, then remove the six screws holding the amp/controls assembly to the Def cabinet. That assembly pulls away and that's what you ship back to Zu.

David, I'd be skeptical about Sean's advice on simple rubber footers. If you truly need greater bass warmth, trial the IsoAcoustics Gaia footers. I found they were outperformed by the Revopods, but in a situation where bass is MIA, they could be the ticket.
I've managed to remove the sub amps and will send them out today. It was a bit of a challenge pulling them out due to the glue tape! I was surprised how tiny the RCA cables from the speakon inputs to the amps are. Flies in the face of the massive audiophile cables everyone puts on their subs these days. 

Perhaps this is actually an ideal time to play around with toe in, as I'll be without the bass output for a couple of weeks. Based on my listening so far, I thought that the Mk4's in my room would benefit from a bit less toe-in, so it's good to hear your input on that, Phil. I'll try your suggestion of some older recordings. I've got some great jazz from the 50s. Interestingly, I asked Sean about this and he said that the ideal positions he has found in various setups with the Mk4's have ranged from toed-in from as extreme as 2 feet in front of the listening position to some distance behind, usually no more than a couple of feet. He estimated 1/10 times directing the tweeters in front of the listener, 4/10 directly at the listener, and 5/10 behind. I always had the Mk1.5's pointed right at my ears, but notice that these new speakers aren't disappearing as well my old ones in that position, with the frequency spectrum tilted upwards (a little too much high end content in comparison to the mids). 

Marc, I was just looking at those footers! The company is based in Ontario, about an hour from where I live, though I think they're actually manufactured in China. Crutchfield carries them and has an easy return policy, so it's a no risk proposition to give them a go. I'm just waiting to hear back if they have the alternative threads (looks like the Zu's are 3/8"-16, which isn't included in the box). 


The IsoAcoustics Gaia footers are rated for speaker weight up to 70 lbs per set. Definitions well exceed that, so the isolating compound may not as effective under greater mass.

An alternative that is effective for me is from Herbie's Audio Lab:

Under Defs you can use either the regular or giant size, and the titanium spike receptor generally sounds best. This simply puts an isolating compound under the spikes.

Hey Phil, 

I actually have older spike receptors from Herbie's that I've been using with BDR cones, similar but different to the one's to which you linked. But in the interest of trying something new, I've ordered the IsoAcoustics Gaia's. I learned they actually come in multiple different sizes. The Gaia III's can only handle 70lbs per set, but the I's are rated up to 220lbs. I managed to negotiate a good deal from a store I've gotten a lot of pro audio gear from in the past, so I'm going to give them a shot. The manufacturer also agreed to send me the correct thread adapters directly. I'll let you know what I think about them!


David, I thought Phil didn't have that right. Yes, the Gaia I's are the ticket. You will certainly hear a different character in the bass. Whether that's ultimately for you...

I did try GSM's subs settings of 46Hz and 9/10 Level, but in the end found I was better off sticking w my current 38Hz and 6.5/10, as guided by Phil.

I may experiment w toe in. I sit 15' from my Zus, and they cross 3' behind me, much flatter than I initially had them, or the Def 2s before.
Hi Marc, when I spoke with Sean yesterday, he also said he generally liked a lower setting around 38-42Hz as well, depending on the setup and type of music (lower for music with deep synth bass, for example). He said he has never had a need to push the crossover higher than 50Hz. I was cranking it up as far as 90Hz to get the bass I wanted, but obviously that began to smear the lower mids.

I'm hoping the adjustment in amp gain, the upgrade to the Lundahl transformers, and the increased gap height from the Gaia's will all yield improvements (Sean did say gap height could slightly increase the bass output). The toe-in may be quite significant as well. You are lucky to have 15 feet, which I'm sure lets the low frequency waves unfold more. I am 9 feet tweeter to ear, pretty nearfield and hard to get a deep soundstage dialled in, though it does make for a very immersive experience. Like front row seats!

Well, lots of positive developments here. I'm not sure why I expected the upgrade from the Mk1.5's to the Mk4's would be easy and instantly gratifying. I should know by now that, as with everything in this hobby, it's a learning curve and a journey to get to musical nirvana...

David, it's totally reasonable not to feel so challenged you can't get it right. I'm envious of those Def4s owners who got things right on the first day, kicked back and relaxed.

Indeed, I've been running my 4s for just over 6 yrs, and in concert w my analog reinstall coming up trumps, I'm FINALLY properly dialling in my Defs.

For me, they've taken big strides re room position, footers, Duelunds and Lundahls upgrades, power cords, fuses, grounding (Sean installed ground posts for me), balanced pwr to subs.

Updated Lundahls, Gaias footers and careful attention to subs settings and toe in could provide big upsides.
Can you tell me more about the power upgrades you are describing?

I've found a simultaneous benefit and drawback of the high efficiency of these speakers is how much they reveal noise issues in my system, including very minor ground loops. I had been considering star grounding the components prior to acquiring these new Zu's. It would be great to include them in the scheme, if adding the grounding posts is something that could be done while the sub amps are in for upgrades...

On other fronts, I've played around with power cords, without hearing really significant differences. At one point, I had Shunyata cords on my previous Definitions, but wasn't sure they were any different to the Zu Birth cables that came with them. Upgraded fuses have also been difficult to assess real vs. perceived gains. Did you upgrade a fuse inside the sub amps?
Keep in mind, the sub amp is a Class D, with relatively low consumption requirements, and the amp has limited frequency duties in this application. You have far more influential opportunities elsewhere in the system to strive for more objective sound than worrying about the power cord and fuse to each Def4.

Totally true, Phil, I was just curious where Marc has gone with his power upgrades. I still have some work to do to reduce some background noise in my audio chain, particularly when using my turntable as a source. I agree that the subs are the least of my worries in that regard. I had wondered, however, if the power cords to the Zu's were introducing the possibility of some ground loop hum (heard through the compression tweeter), as they are currently plugged into a different circuit. Solving grounding problems has yielded tremendous gains for me in the past, on both the listening and recording ends of the spectrum. David
Yes, you should solve grounding problems. You're hearing hum through the Radian compression tweet? That's strange. No, the Zu Def power cables aren't introducing ground loop hum. But you probably have induced some ground loops in your system connections, and it's likely your ground points are not all on the same plane. Solutions are always elusive. If the sub amps' power is contributing, it's not the cords, it's having them plugged into a different circuit. By chance, do you have cable TV among the sound sources into your system?

Also, is any portion of your system balanced or running balanced or XLR cables? Mixing Bal with SE can also surface ground problems. What is your analog chain (phono-pre > pre) and is there an SUT in the mix? How many (if any) of your components have ground-lift switches? Do you have any passive components that are not separately grounded? What's your phono cartridge and tonearm, and how closely are they placed to anything else with a substantial power supply?

Just some questions for which answers may narrow your suspects.

David, Phil is right. Do not expensively spend on subs ancilliaries until you've got basic setup right. At that point if you're happy, and want to go on, my personal experience is that choice of pwr cords and fuse makes a difference.

I'm stuck with a little hum in my Zu sub amps that I'm still searching for a solution.
Yeah, I'm not too interested in spending much on power cords etc. at this point. I was just interested in the grounding posts you had installed on the Zu's and your overall power management scheme, as I've struggled with some hum and slight buzz in my system. I do have the subs on the speakers on a different circuit, which I realize is an issue, but was more practical given how things are arranged in my listening room. Also, I misspoke when I said the ground loop hums are coming out the tweeter... obviously they are oscillating at 60hz. I actually only have a small amount of 60hz hum present in the FRDs and not audible in the sweet spot. I do, however, have some hiss and buzz out of the tweeter when the phono is selected on my turntable. I was able to reduce the buzz significantly by running a ground wire from the lamp above my turntable (on a dimmer, which is a likely source of the problem) to the turntable itself and then to the preamp. I had considered a star grounding scheme for the whole rig, which worked wonderfully to lower the overall noise floor when I did this for the analog mixing setup in my home recording studio. In such a configuration with grounding posts on the speakers, I could then lift the grounds on all but one of the components for a single path.
Request ground posts for Sean when he looks at yr amp modules.

I'm sure you'll like the Gaias footers, some mileage there.

The other thing to say David is that eight 10" woofers versus two 12" was always going to present a difference, esp rear facing versus down firing.

I think you need to approach the 4 as a different spkr to the 1.5, blank canvas approach on set up.
Very true, Marc, I'll start from square one with setup once the amps are back. I really appreciate all the help you and Phil have given me here. David
My pleasure David, I hope it helps. I've been masochistically flying the Zu flag for 6 long years on What's Best Forum, mainly receiving brickbats in return LOL - so, its great to try and help by conversing with a fan of the brand.

Certainly I'm sure Sean and Phil can bang their heads together and get you in the right direction.

Be patient. I have been, for 11 years now LOL.
If I can get detailed answers to the questions I raised a few posts back, I might have more suggestions. -Phil
Hi Phil, 

Here's some answers to the questions you posed:

-- no cable TV in my sources
-- no balanced / XLR cables
-- preamp is a Cary SLP-98 with integrated tube phono stage, no SUT
-- no ground lift switches on any components
-- no passive components
-- cartridge is a Clearaudio Virtuoso v2 ebony, tonearm is a VPI JMW-9
-- there is a lamp situated above the turntable, which was causing buzz when turned on, but that was mostly eliminated by running a ground to the turntable and then preamp. The lamp is off when listening anyway. No other power supplies near the turntable. 

Interestingly, Sean sent me this response about grounding the sub amps in the Definition (I'm sure he wouldn't mind me posting this): "When I rebuild the amps I float the audio ground, that is one of the benefits of the transformer. Another thing I do is to better float the AC transform, much of the hum you notice is coming from the mechanical singing of the AC transform, the power supply is linear so it’s a big piece of the kit in size and mechanical effect. Things should be much quieter when you get them back." 

Well based on that reply, I need to contact Sean about solving my subs hum issues as well.
David, Sean has emailed me to say the solutions he has for you may well suit me too. Great customer service.
That's great, Marc! Maybe this thread will end up solving problems for both of us :) 
By the way, I've also installed Isoacoustics Gaia I's to the Definitions. Can't wait to get the amps back to take them for a test drive! Sean said it was about a week turnaround. 
Looks like the sub amps are headed back to me. I'll report back in when everything is up and running. Did you end up deciding to get your amps upgraded to fix the hum issue, Marc?