Advice on replacing 12 in woofer in Martin Logan Monolith III's

I'm needing to replace the 12 inch woofers in my Martin Logan Monolith III's.  I am running a dBx 260 active crossover so I can control x-over points and output.   Any suggestions? 
Yes, the driver ML commissioned Eminence to make is rubbish compared to the rest of the speaker, it has a one note bass, I’ve got all the T/S parameters on it, not good.

The ACI-SV12 ( Meniscus SW1284 ) I matched and replaced them with are miles better, for that box. You’ve got to find a used pair though. I could find much else that suited.

Heavily stuff the box so you get a lower Q (.6) to equal the speed of the esl panels, cross the SV-12’s over with 24db active at 130hz,
Keep using the ML’s own passive 18db xover to the ESL though, as it sound better than active. You’ll then have a speaker that comes close to the ML Neoliths.

Cheers George
They were too expensive to make that’s why they were stopped, but many were sold, dig deep for used ones.

The SV-12 were only one I found that went low enough and up 130hz, also known as the Meniscus SW1284. There were some that had as good LF performance but they weren’t to good up at 130hz, meaning they had to be cut off lower at 100hz and the ESL isn;t good down that far.
Here are the T/S specs which ended being perfect for the Monoliths sealed enclosure.
Especially if you used the SV-12 Third Order Sealed System which is just a 2500uf non-polarised cap in series with the + line, takes the +2db bump out before it rolls off at 25hz and takes it a little lower to 20hz, very tight and flat and protected against amps going dc.

Nominal Diameter: 12",
. Impedance: 4 ohms,
. Range: 15-500Hz
. RMS Power: 350 watts,
. System Power: 500 watts,
. Sensitivity: 90 db
· Magnet mass: 1930 grams,
. BL: 8.9 Tm,
. Fs: 17.5 Hz
· Qms: 8.35,
. Qes: .41,
. Qts: .395
· Vas: 250 liters,
. Cms: 1041.67,
. Mms: 99 grams,
. Mmd: 93 grams
· Xmax (linear one way): 12.87 mm,
. Voice Coil Diameter: 2"
· Sd: .0460m2
· Voice Coil Height: 31.75 mm,
. Air Gap Height: 9.652 mm
· Voice Coil Inductance: 1.45 mh,
. Voice Coil Resistance: 2.97 ohms
· 12 3/8" overall width,
. hole size 11 1/8" Flange depth 3/8" (1/2" with gasket)
· Overall depth from front of flange to rear of magnets: 6"
· Driver displacement: 2 liters, (130
· Heavy duty, gold-plated, high-pressure solder less terminals

Cheers George
ACI made a passive sub kit called "ACI Titan" that used the SV12 many were sold, I believe through Madisound, maybe look for a used pair of these for sale.

Cheers George
  If you can get the woofer cabinet dimensions so that we can get cubic ft or liters,  I'll take a look around... If I understand the Monolith III correctly, it is 89db sensitivity and the woofer is passive (not powered)....
Being 4 ohm,  you may find a good automotive woofer... the competition woofers are normally made for small boxes.  I suspect we'll find something decent.  Getting down to 28 hz in a small box will be the biggest issue. Get me box dimensions and we can all take a look around.
This was years ago, I did volume it using bean bag fill because it's an odd shape looking for the right driver, but I didn't log it down, don't hold me to it but I think it was 3 cubic feet.

Cheers George  
Post removed 
if it indeed 3 cu ft... this is expensive ($245 each), but ran curves F3 in QB3 alignment is 25hz 
Peerless  835017
If the box is a smaller, there are others that can work for less money.
again, based on 3 cu ft......

ScanSpeak Discovery 30W/4558T   This looks good.
Peerless 835017

I think I remember looking at this, but remember the the distortion was better at 150hz on the SV-12. But the main thing for memory that turned me off it was that it’s only 83db where the SV12 was 89db and the ML panels are also 89db.

Cheers George
ScanSpeak Discovery 30W/4558T

This is better 89db, but investigate the distortion at 150hz and 1 octave above (300hz) compared to the SV12.

Cheers George
I've emiled ML to find out the cubic size,  but who know if they will respond. 

SLS 830669
FR too high for me.

I was told by a mate here in Australia who also wanted them for his Monoliths after he heard mine, that this guy still had the SV12's but he wouldn't ship to Australia.

Cheers George
George do think the ScanSpeak Discovery 30W/4558T would be my best option?  I dug around looking for some SV12's, no luck yet.
You'll have to find what the distortion is like at 150hz and one octave above 300hz to see if as good as the SV12's stevepa, why not get in touch with the guy how's got the SV12's in the above link.

Cheers George
Hi guys,  sorry on the Peerless recommendation, I don't know why I missed that it was 83db sensitivity.
@stevepa    on the Peerless SLS 830669, it will go down to about 50 hz in a sealed box. I would not mind porting the box on the right driver, but this driver really wants a large box ported. Also, it has a very cheap frame. That does effect sound quality.
In 3 cu ft, the Scan Speak will work well. The Monoliths published crossover point is 125hz. I believe stevepa said that he was crossing electronically. Good excursion limits, however, with a qts of .32, you must vent or port your box with this woofer. If you do so, it will go be 3db down at just about 27 hz in an sb4 alignment or 24 hz in a qb3 alignment.
As it stands, I'd run this way without worry of distortion.
Of course box size could change it all. 

Just thinking,  if you are using electronic crossover and biamping, why do we care about matching sensitivity or impedance? 
I still think that 83 is too low, you'd have to drive it pretty hard to keep up,  but you could certainly use anything from 4 to 8 ohm from 87 to 92 sensitivity. 
Based on that, I will expand the search.
you must vent or port your box with this woofer
The Monolith I was ported and the bass couldn’t keep up with the speed of the esl, that why with the II and the III they went sealed, tighter faster except they used that junk cheap driver, I posted it’s T/S here.

The one Jon Fo used could work Peerless 830699 but it’s FS is much higher (double) that of the SV-12

Cheers George
Thanks for all the input.  I have reached out to ScanPeak  for more data on the Discovery 30/4558.  Also inbox the guy that had the SV12's for sale,  but haven't heard anything. Even though the post was a year ago, I think maybe he still has them.   I will be putting a sub with the setup,  so I don't need it to down real deep.  Question:  what would be the ideal  crossover point for the ELS  running at three-way?

Even though the post was a year ago
My friend contacted him only 3 mts ago, still had them.

Cheers George
The MMS of the 30W/4558T00 is the moving mass, it’s some 36g higher/heavier than the SV-12 which says to me it’s not going to be as good at 130hz and 1 octave above, fine for below 80hz but the Monolith panels won’t take to that low frequency too well.

Cheers George
That's good news!   Hopefully he will respond to my inbox.  Thanks for all your help George!

just ran curves on the SV12 in 3 cu ft sealed.... It'll be 3db down @ about 31 hz with a qtc of .79
Thanks for the info Tim.   Finding that driver is the problem  :).  It seems they are very hard to come by so I'm trying to figure out what the next best thing is. 
just ran curves on the SV12 in 3 cu ft sealed.... It'll be 3db down @ about 31 hz with a qtc of .79
I can't be sure about the 3cu ft as I didn't save it, I think with a heavily stuffed ML box that came down to around 20-25hz and around .6 which was I was told by Neville Thiel (rip). Sliightly less efficient,  but that was fine as it was active xovered.

Cheers George
Congrats on your subwoofer purchase @stevepa   if you would, just go to the back of the box and get the woofer box measurements, we'll get very close on the cubic footage.   I'd hate to see you buy the wrong woofer.  Overall, I don't like over stuffing a box, (unless your box is too large) it actually raises QTC by limiting air flow, the woofer acts as though it is in a smaller cavity. A couple of other comments that were a bit off... Porting not keeping up with a sealed driver....Porting a driver in itself is not slower than a sealed driver... If optimally ported, speed should be fine...   Of course similar to a sealed box, if you seal a driver to get a qtc of say 1.1, it will get boomy on you, if you seal it for a qtc of .6, it will get lean... on a ported box, you can easily affect the curve with the port. Overall though, I would also want a sealed driver if I could find it. I prefer the rolloff and they don't go out of spec as easy over time.  Lastly a quote on driver cone mass slowing down the driver... Yes, if cone has less mass, it will be faster, but that would only be if 2 drivers had identical motor structure... with different motor structures, the cone mass alone cannot determine the speed of a driver. In fact, If you had a QTS of say .4 and you lowered the mass, qts would drop and sensitivity would rise, the driver would want a smaller box.... I have learned a ton from others on this site, so I don't want to come off rude or condemning,  but I do want you to see the facts.

I don’t like over stuffing a box, (unless your box is too large) it actually raises QTC by limiting air flow, the woofer acts as though it is in a smaller cavity.
Neville Thiel rip, said the opposite to me, it increases the apparent box size as seen by the woofer and lowers the Q.
Gale GS401 and Infinity 4.5’s were a classic cases of one of the fastest/tightest detailed bass at the time, heavily stuffed, the drivers had to be pushed down while tightening the mounting screws, sure they were probably a bit harder to drive. 

Cheers George
again,  stuffing in itself does not dictate speed. 
A lightly stuffed box is what a woofer is normally spec'd at, as you add stuffing, a medium stuffed box, does give the woofer the illusion of a larger box by slowing down the flow of air inside the box, the air has to filter through the stuffing.  When you over stuff the box, air flow cannot pass through and has the opposite effect.  Air can't pass through at all and the woofer reacts as though it is in a smaller box.....Maybe we have a simple question in terms about the amount of stuffing (what is lightly stuffed or heavily stuffed) As I stated earlier,  all things being equal,  I prefer a sealed box sub myself. Sorry,  but this is not opinion.  It works this way.
In a sealed box woofer, I have many time added mass to raise qts so that a woofer would go down better in sealed box.  The key there is "does the driver have enough motor & suspension to support the mass"  I hope this helps,  Tim
I will still look for the SV12's but if I can't find them what driver would you guys recommend?  I'm just not as knowledgeable on driver specs.  Being that I 'm running a SVS sub,  I need a driver that is a good mid-bass.
@georgehifi     Hi George, it sounds like you and I are just mixing terms of what is lightly, medium or overstuffed, but overall are on the same page.
@stevepa     Steve,  get the outer dimensions of your woofer box on the back of your monoliths.  We can get very close to figuring its cu footage.  We need that to stop guessing. 
There were other woofers that did not go down as well, so if you are happy with that, using your New SVS sub, you'll be fine.  I would only worry about resale later. I'd want a driver that went down well. 
Get the dimensions and we'll go from there.
So the box is not square as George indicated, it is 25 inches wide and tapers to 19 inches at rear.  24 inches H and 11 inches deep
Ok,  assuming a 1 inch front baffle and 3/4 inch everywhere else, you're looking at 2.469 cubic ft... so if we model on 2 1/2 cubic ft, we'll get accurate results.
I have many time added mass to raise qts so that a woofer would go down better in sealed box.
Adding mass to the cone does lower the FS, but you didn't mention it also raises the distortion (slows down it's reaction time) at the higher end of the woofers capabilities.
You give to one end but take away from the other, there's no free lunch.

Because it's such an odd trapezoidal type of shape, I'm positive when I bean bagged filled the bass chamber to do a proper volume check, it was more than 2.5cu ft.

Cheers George
Just listen to the Eminence ones till you get some SV-12's, two of us, Jon Fo and I both looked for a replacement for a very long time and the SV-12 was the one.
Jon Fo later did raise the xover point to some 300hz? to take away some of the power frequencies from the ML esl, and then used the Peerless 830699 for bass/mid/upper bass but I think he then used big external subwoofers as well to make up for the Peerless no being able to go down.

Cheers George    
Sorry George, Not sure where all of your info comes from, but adding mass in alone does not add distortion... In fact if the mass is a stiffener, depending on cone, it could decrease it.
@stevepa    Steve,  George and I could take you down 2 different roads.  To be fair to George, I do not have his direct experience with your speakers, but in fairness to me,  I come from a back ground of driver and speaker design.  I speak from experience and an understanding of how all of this comes together. 
 The good news is that I do agree, that a good sealed woofer is the way to go.  I have not found one that is really a great match for your Monoliths.... On the other hand, I have found 1 Dayton Pro, that would work very well indeed if you would be willing to port. It would be a very viable alternative.  It seems George is questioning your measurements,  if they are correct, you are at 2.5 cu ft.... Bean bags would decrease it. 
  I hope all of this helps,  Tim
If you are up for it,  Jon Fo's idea for George is not bad either to raise the crossover point,  The Good, removing the mid bass (100 - 300hz)  off the panel, cleans up the mids and make for cleaner midrange.... The Bad is that you have a crossover point in the vocal region. 
Its a trade off, but easy enough to do if interested.
Also,  I have looked for this, but have not found it... the best single woofer that I know of for your application is an Tonegen 1259, also known as NHT 1259... Madisound had done a close copy called the Madisound 1259... so if you are looking around for the SV12, You could add this to your search... Good Luck,  Tim
Sorry Tim but you clearly have no experience with mating esl’s which are very fast (low mass) compared to a dynamic drivers (much higher mass), as there are big problems at the xover point to get them to seamlessly sound coherent and it is an art to get them to do so. A low FS for LF, and a low MMS for speed at the xover point is a good place to just to start, this is why the SV12 is an ideal choice.

Adding mass to a driver just to get it to go down further will create problems up higher exacerbating the problem where this xover point is most critical to have both esl and driver sounding/blending well to be coherent.

Over here a Monolith I (ported) sounds worse in the (above area) and is reflected in it’s used value around $3k.
A Monolith III (sealed) used fetches between $10k-$12K why would anyone want to port this and "maybe" get a worse sound.

BTW: Bean bag (pellet) fill (beans being the operative word) will give you an exact volume measurement of that weird shaped enclosure, as it takes everything inside the enclosure into account.
I hope your not thinking, leaving the fill in there as stuffing??? As that would be stupid. It’s a precise measuring tool, then to be taken out and put into a symmetrical container to work out the correct volume of the Monoliths weird shape enclosure volume.

Cheers George
If you do raise the xover point to 300hz you get the problem that many speakers have of having two drivers doing the critical midrange. This is even much more accentuated with ESL’s.

This is why a Monolith with a 150hz xover sounds so much better in the critical midrange than a ML Summit with a 300hz xover. The reason Jon Fo went this way is because he uses 4 x Monoliths with external subs is for HT use with all that processing and that critical midrange xover in HT is not as big a problem than just hi-end stereo use.

Cheers George
"Sorry Tim but you clearly have no experience with mating esl’s"

But what I do have is a knowledge of how it works and why.  I do understand blending panels and a woofer...
I know that you guys don't know me from anyone, but when Sumo designed the Aria speaker, they did call me for advice and way back when,  The very company that you are speaking of Martin Logan did call me and ask me to come interview with them... which I declined. 
I'm old and I've been involved in speaker design since I was 19. So rather that turning this worst and telling you what you don't know, I will bow out....
@stevepa can easily private message me if he needs help. 

That will make quite a difference.

Best to lay the Monolith on it's back take the driver out and stuffing, fill with bean bag styro bean fill, empty all into a known litre/gallons container (I use a 20lt contaner I had) to get the exact volume.
Then Google litre/gallon to cubic ft and use the calculator to find the exact cubic ft on the Monoliths bass enclosure size include bracing, esl power supply, wires ect.

Cheers George
You would think this  measurement would be on the Internet. I've looked at MartinLogan site and around the net, but can't find it. 

No it's not, as I found. You'll just have to do it yourself as I did, wish I saved it all those years ago.

Cheers George
Steve, here is Jo Fo's front speakers, as you can see his centre something his subs are not in the picture.

Cheers George 
With a lot of advice and help from Tim (Timlub) I purchased a Dayton 12 inch driver. Since I'm running a dbx 260 active xover, I was able to to dial in the xover points and the slope,  it sounds amazing.  Also I added a SVS sub, which really brought out the lows.  Still have some room EQ I need to deal with, but overall it sounds awesome. Thanks Tim!