Review: Pass Labs X-350 Amplifier

Category: Amplifiers

Talk about taking an expensive chance! I bought this amplifier solely on the basis of the advice of a couple of audiophile friends of mine, and on the recommendation of Stewart Marcantoni of A Sanctuary for Sound. a Port Orchard, WA dealer whose advice has proven to be consistently reliable and accurate. Two weeks after placing my order, the amp arrived on the doorstep of my North Idaho ranch. The first thing you'll notice about this amplifier is that it's a brute: 150 lbs w/carton. I managed to heft this package up a long flight of stairs, but it’s helpful to have a friend handy to assist you with unpacking and placement. A sturdy hand truck is also a Good Thing to have, especially if you have to negotiate stairs as I did.

A Shocking Development
Since I had ordered the amp with the IEC 20 amp power cord option, I hadn't reckoned that my conventional AC outlets were the wrong configuration for this type of AC cord - NEMA 5-20 outlets are required instead. Doh! So, I had to do a retrofit to my dedicated AC outlets. This inadvertent upgrade was easy to do and resulted in better quality AC outlets. Upgrading to to these outlets seems to have reduced the overall noise level in the system. Cheap tweak, come to think of it: $20 for four pair of these NEMA 5-20 outlets at your friendly Lowe’s hardware store.

Looks don't hurt
The X350 is a gorgeous piece of industrial design. Massive - and I do mean massive - face plate and fascia, with one Cyclopean bias meter dead center on the front panel. Lit up, the meter face assumes a beautiful brilliant blue hue, with the same Cherenkov-like hue spilling from the amp's ventilation slots. The meter’ serve to indicate when the amp transitions from class A operation to class AB; this event occurs as power demands exceed 90 watts, according to the manual. Sturdy rack mount handles on the back of the X350 are another Good Thing, considering its sheer mass. Five soft rubber feet support the chassis; the somewhat soft rubber makes it difficult to slide the X350 on a smooth floor (and just forget about carpet!). However, the composition of the feet suggest that there may well be some damping and isolation benefit there.

The back panel features XLR and RCA inputs. I have used it so far only with single ended RCA cables, so I can’t comment on the differences between the two input options. Speaker connections are made via two pair of five-way binding posts; each are enhanced with sturdy plastic wing nuts that allow a firm connection.

Initial impressions
Right out of the box, the X350 beat my Krell KSA 250S hands-down ,and my Bryston 4B ST wasn’t even in the running. My initial impression was that the X350 delivered a far more organic feel to the music in that there was more of a sense of seamlessness and continuity. Each passage that I heard seemed to have more ‘weight’ somehow. The amp delivered scads of inner detail – I could actually hear the impact of fingers on keyboard on good piano recordings, for example. Voices are to die for - whole, deep and real. Even without the lengthy break-in period recommended by other reviewers, the mids on this amp are pure and breathtaking.

Listening at length
There’s a crystalline clarity up top that I believe is more a reflection of the X350s ability to provide additional information in the form of detail. But this detail is delivered without a tilt towards the top, and without a hint of grain. The five foot ribbon tweeters of the Magnepan MG20Rs are mercilessly revealing, but at the same time they are quite possibly the best high frequency drivers on the planet. At no time did they sound hard or harsh - unless that was present in the recording. There was no sense of solid state hardness or the infamous 'MOSFET fog' heard in other MOSFET designs.

Some reviewers have reported a initial mid-bass emphasis that disappears after 200+ hours of run-in - I didn’t hear it, and neither did my wife, Candace. When it comes to deep bass performance, I would agree with other reviewers in that this amp isn’t the last word in Krell-like low end slam, but I have a feeling that it’s more a matter of the fact that the X350’s doing a much better job of sorting out low-end detail and definition. The X350 appears to be presenting more information in the low end than then did the Krell. I can hear the transition from one low-pitched note to another, something that simply wasn't anywhere nearly as obvious with the Krell. Low-pitched timbres emerged with distinct identity where they were not evident before.

The X350 delivers a better presentation of the entire dynamic envelope, and this is key to what makes this amp better than many of its higher-priced competition. Smaller, lower-level bits of musical information aren’t masked by larger events, so you’re actually hearing more of what’s there. And all of that is kept in perfect proportion up and down the dynamic range of the performance. This is what I believe lends the impression of wholeness and weight to each musical passage.

Power to spare
Then there's the sense of sheer, effortless power. The X350 exhibits a sense of utter and effortless control over the MG20Rs, and at any level. The MG20Rs can be driven to ridiculously high levels, and the amp never loses its composure or alters its spectral balance. I could clearly hear the Bryston 4B ST run out of gas on piano passages in Mozart’s Concerto No. 21 in C major (Reference Recording RR 68CD) played at concert levels. The Krell KSA 250S was less prone to this, but I could still hear it give up. Not so the Pass X350.

I must run the X350 really, really hard in order to get the blue-lit front panel bias meter to budge off dead center. That takes the full chorus, orchestra and organ from the last movement of Mahler's 8th (Solti/Chicago Symphony) at realistic levels to make that bias meter move. So it would seem that the X350 is running within that class A regime (up to 90 watts) most of the time. This says a lot about this amp's reserves and current delivery, especially since it's driving the MG20Rs - a notoriously inefficient speaker that benefits from a lot of current and iron-fisted control. In sum, this is absolutely the most musical and utterly transparent solid state amp I've ever heard. There may be tube amps that might offer a shade more in the way of micro dynamics or inner detail, but they do so at the trade-offs of less power, more heat and the sheer dollars spent to get there.

As the X350 warmed up and broke in, the soundstage began to unfold and deepen. Placement side to side and front to back is nothing short of phenomenal. If anything, the X350 has improved after more than 300 hours or so of break-in. It seemed to improve in subtle ways almost day by day, becoming even more detailed and seamless from top to bottom. Low end took the longest to 'settle in', and was the most subtle of the changes, manifesting itself in a degree and clarity of low frequency low level detail that I simply hadn't heard on any of my CDs and LPs until then. It's a funny notion, to think of deep bass as being both strong and delicate at the same time, but that was the overall effect. The X350 made acoustic bass 'action' and fingering come alive in a wholly unprecedented way, imparting a 'right there in the room' sense of reality to well-recorded acoustic instruments in particular. Check out Michael Gettel's San Juan Suites II and Pat Metheny and Charlie Hayden's Beyond the Missouri Sky. Absolutely lovely piano and acoustic bass duos and a real system test. Midrange appeared to, well, relax is the term I'd use. As it broke in, the sound space that this amp delivers has expanded beyond its initially impressive sense.

Still, there’s something that I can’t really put my finger on when trying to describe what this amp does (or maybe doesn’t do). Candace may have got it right when she said that everything seems alive now. For me, it seems as if everything I listened to was somehow more compelling, more worthy of my attention. Perhaps it’s that the X350 allows more of the emotion and what we might call the ‘inner truth’ of each performance to come through. While there's nothing euphonic or theatrical about this amp, it somehow manages to come across as the most musical design I've ever heard. If money were no object, I suspect that I'd move up to the X350s more powerful cousins, the X600 or the X1000 monoblocks.

Other reviewers have consistently said that this amp really takes around 300-400 hours to truly settle in. Our listening experience suggests that this is an accurate perception. The overall presentation became deeper, more articulate and more effortless - it’s as if the system is doing a slow dissolve and leaving just the musical event in its place. And that's what it's all about, folks.

Associated gear
System used for audition
ARC LS22 preamp - retubed with NOS 70's Russian 6H23s
Classe CDP-1 CD player
Bel Canto DAC-1 24/96 upsampling DAC
Synergistic Research Designer’s Reference Digital link
Linn LP12 / Ittok LVII / Dynavector 10X4 turntable/arm/cartridge combo
Magnepan MG20R speakers w/XO20 crossover.

Cables and interconnects
Cardas Golden Cross interconnects
Cardas Golden Reference bi-wire speaker cables
4 pair of .5 meter Audio Magic Spellcaster II jumpers (to go from the XO20 crossover to the MG20Rpanels themselves)
Electra Glide Fatboy power cables.
Bybee Signature Pro power purification.

Similar products
Krell KSA 250S
Bryston 4B ST
Levinson 335
Nice job with the review! Let us know how many hours it actually takes to completely break in.
Very nice review! I have two X350s and love them. Please use the balanced inputs, you will be even more impressed.
Nice review. Makes me want to run out an order one!

I am a little concerned about simply replacing the 15A recepticles with the 20A ones unless of course the supply line is actually 12 gauge copper or better and with a 20 AMP breaker. I doubt the amp will ever be pulling enough for it to be a problem but there is a reason Pass put the 20A plug on the X350. While in practicality there unlikely will any problems, I'm sure the buildng code and your homeowners insurance may a have a different view. FWIW.
Congratulations on making an excellent purchase and writing a very nice review. You have a nice writing style that is both comfortable to read and informative at the same time. You did a nice job of covering all of the various aspects of the unit and segmented each part so as to make it easy to read i.e. "good flow".

As to the amp itself, i'm sure you'll love it even more as you get more time on it. Nelson's amps are very "real", "organic" and "natural" sounding. There is just a sense of "musicality" that you get along with all of the detail that is hard to imagine and / or find in other products. It truly is something that one must hear and experience in a well set up system to be able to relate. I think that your review did a nice job of "trying" to explain that : )

Hopefully, both you and your wife will enjoy the system for years to come. Sean

PS... I see you listed a Levinson amp but did not make mention of it in the review. Any thoughts / comparisons in that regard ?

PPS... Have you been turning the amp off and on as needed or leaving it turned on ?
Great review! It makes me want to trade my Krell 650MC's for this amp. Want to trade? Just kidding! My neighbor had that amp for awhile and now I wish I had tried to demo it. Enjoy the amp and the music. I can't help but wonder if that tube preamp helps the great imaging and inner detail.
Killer review. Sounds like you found the amp for you. Pass does make some great gear.
Fantastic review. I may have to give these a try someday.
Hi, folks -

Thanks for the commentary and the advice. I'll try to respond to all that I've see nso far:

1. Break-in time. All I can say is that, after 250 hours or so, we didn't notice any further changes in the X350s attributes that we could directly ascribe to the amp itself. Other changes, such as the upgrade to Cardas Golden Reference bi-wire speaker cables would have obscured any further judgement on that score. BTW, these cables made a substantial difference, and I'll review them after I've had a while longer to live with them.

2. AC Receptacle change. Thanks for the inforamtion WRT the receptacle change. As part of an on-going remodeling project, I did twin 12 gage 'home runs' to the breaker panel, so I'm comfortably within code for these outlets. However, I'm thinking of isolating the music system AC feed to a separate subpanel with a SOLA isolation transformer in line to screen out any possible interference and noice for mthe rest of the AC distribution system.

3. Amp comparisons. I listed the Levinson 335 becuase I had auditioned it against my Bryston 4B ST and a Krell FPB 300. At the time, I had MG 3.5Rs, and a friend of mine was consideing the Krell vs the Levinson, so we set up a weekend listening session. The results were that the Levinso and the Krell were noticeably better than the Bryston in a number of areas, but not by a huge margin. Longer-term listening might have given us a better picture, but we got a pretty good idea of each amp's character. The Levinson was warm, detailed and buttery smooth. And completely uninvolving. Too polite, was our assessment. Some folks like that. The Krell was exciting, compelling and theatrical. Lots of 'wow' factor. The 4B ST was more 'Krell-like' than the Levinson, but less refined. That was the upshot of our brief experiment.

4. Leaving the amp on or off. Generally, I leave it on all the time in the fall and winter seasons - along with the wood stove, it helps warm the room . During the warmer months, I leave it in 'stand by' mode, but turn it on an hour or so before doing any serious listening.

5. Preamp and imaging. Well, I've always been of the opinion that a great solid astate amp combined with a great tube preamp are complimentary in that each plays to the others' strengths. It gets me the best of both worlds, IMHO. This notion can make for some interesting discussion and debate, and no doubt there are always exceptions to this rule, if I can call it that.

6. Balanced vs unbalanced inputs. Thanks for the advice. As soon as budget allows, I'll replace the preamp-amp connection with Cardas Golden Reference XLR cables and I'll document the results. BTW, what do you notice comparing balanced vs unbalanced inputs?
I purchased an X-250 and a X-1. I heard the LS-22 and if I were you, I'd try some more preamps, unless you think the tubes significantly improved the standard tube sound.

Great job!
It was a pleasure to read your beautifully written article. Yours looks like a very enjoyable system (I've always loved Maggies, but doubt I've ever heard them driven as well as yours must now be), and located on a ranch yet - great combo for environmentally uncontaminated listening, I'll bet. I see you only have a couple of answers on the forum otherwise, both from pretty recently, so I look forward to reading more of your input in the future, and bid you welcome!
Hi, Zaikesman -

Thanks for the compliment. It's taken quite a while and no little amount of experimentation to get the system balanced so that all of its elements harmonize and synergize.

And you're right - the location doesn't hurt, either. Our place is located on 16 acres straddling a 4000 foot long soft field airstrip. The house is a 60' X 100' pole building that we've renovated into a lower space comprised of approx 3000 sq feet of shop space and a 1200 sq ft aircraft hangar. We remodeled the upper space into something more suitable for a family of four. The original configuration of the living was a little strange, and it took a bit of work to get it comforatable and more usable. But it had some interesting features - an all-tile main living room, a kitchen done in Italian granite, and an small bathroom that would have been nothing special except that it was done in Italian granite and travertine marble. The previous owner is a third-generation Italian stone mason who built stone-cutting machinery in the shop spaces below. Hence the tile, stone and granite.

It is quiet out here. My lifelong ambition was to be able to live where the only noise I could hear at night was the wind in the trees. This would be it. While the listening environment isn't completely ideal, overall, everything sounds pretty darn good.

As time permits, I'll publish reviews of the Cardas cables and the Accuphase DP-75 I've recently acquired. I've also got some plans with respect to MG20R xover mods and speaker frame stiffening. I'll be sure to publish the resutls here.

In closing, I'll say that if I've erred in any regards towards system configuration, it's always been towards the musical over the theatrical or analytical. Music is about communication, emotion, the pictures that it makes in your head and the way it inspires your body to move. Anything else is just special effects.
Hey, Bigkidz -

You're right about the preamp. Now, I just run the DP-75 directly into the Pass X350. The LS22r is used only for phono and tuner listening.
Excellent review. I bought an X350 to drive my Wilson 5.1 speakers and loved it. Upgraded to the X1000 monoblocks which were even better. The Pass power amps are really something!
Thanks and congrats on a most excellent upgrade. Given the efficiency of your Wilson 5.1s, the amount of headroom you have with the X1000s must be truly breathtaking.

When my situation improves, I'm looking to move to a pair of X600s. In the meantime, the X350 continues to amaze me with its transparency and power. My MG20R crossover upgrade project is on the back burner for now, but I have most of the parts I need.
I did extensive listening to the X350. It was a well burned in unit from a dealer. As good as it is, there is still a brightness and shrillness that rears its head when you play less than perfectly recorded material at it. None of the tube amps I used did this, and I do not believe it was just a matter of the Pass revealing more than the tubes - I believe it is still a solid state artifact, whether it be some form of intermodulation distortion or some other transistor distortion.

The Boulder 1060, which is more revealing and detailed, and generally much superior to the Pass, does not do this on the same recordings. Yes, the Boulder is a $19,000 amplifier.

But for my tastes, I can't bear to listen to what this amp does to second rate material.
Curious - Based on your experience, it doesn't seem as if you were listening to the same amp that the rest of us have heard. Now I'll grant you that the Pass X series amps are not tolerant of problems or poor synergy elsewhere in the system. But I've got some of the best tweeters on the planet in my system, and I'll tell you that the X350 is most assuredly neither shrill nor overbright. It is not romantic, either. Poor recordings sound exactly like what they are. The highs on my system are open, clear and extended - my wife and I and our friends can listen through our music collections for hours at a time on my system without the sort of fatigue that would surely result if your observations were the rule rather than the exception.

Sounds to me as if there may be some issues elsewhere in your system. And that may simply mean that the Pass is just not for you.
All righty Ward, my Pass X-350 was delivered about an hour ago. Based on your review and some others in other forums, I made the decision to buy the Pass over Krell or Levisons of similiar outputs. This is my first venture into the realm of seperates or higher end product so forgive me for sounding like the novice that I am. The X-350 is downstairs still sitting in the double box as I am going to have to wait for a friend (or two) to come over and help me lift it out.

On to the reason I am posting in your thread. I am looking to marry this beast up to a McIntosh C2200 tube pre-amp which I have not yet purchased. I will be playing SACDs with a Shanling CD T-200A and into some very old Polk Audio SDA 1s. These are next to be upgraded. I am thinking of moving to Martin Logan Aeons. I am really looking more for your opinion and reccomendations here about what pre-amp to connect this beast to. Since the last post was almost two years ago, I hope you are still watching this thing.