Plinius SA250 Mk.4 vs. Pass Labs X350

Anybody compare these two ss amps? I'm considering upgrading my Plinius SA100 Mk.III amp. My associated equipment is Audio Research Ref. 1preamp, Meridian 508.24 CDP, Dynaudio Contour 3.3. Using NBS Monitor I XLR interconnects, NBS Monitor III speaker cable. Your thoughts/feedback are greatly appreciated.
If I were you....I'd pick up a second Plinius SA100 Mk III and run them as monoblocks. I'm in the process of deciding between an SA250 or 2 SA100's as monos. I'm leaning toward the monos!!
I like Plinius, but look at it this way. The Pass is unquestionably one of the best amps out there in it's price range and holds it's value as well if not better than most of it's competition. You cannot go wrong with the Pass no matter how you slice it.
Consider this.......
Go immediately to an Electrocompaniet dealer and listen to an AW250R amp. While you're at it, check out the AW180 mono blocks. Since you like the sound of pure class A amplification, I must tell you that I would not be surprised if you posted back later that you took my advice and became the ecstatic owner of one of those amps I suggested. Hell, if you can afford an X350- consider the Nemos. My FAVORITE amp regardless of cost or manufacturer.
Good luck!
I have owned Plinius (SA-100 Mk III) and currently own Pass (Aleph 4). I have heard the X-350, but not the SA-250 Mk IV. I would say that Pass unquestionably makes the better amp on the basis of detail, soundstage, neutrality, high frequency and low frequency extension. This was clearly evident, but the Plinius had such marvelous synergy with the other pieces in my system, it makes this comparison a much tougher one than it should otherwise be. The Plinius has that iron-fisted bass (although it is not that deep) , great dynamics, and a marvelously clean midrange, upper midrange and high end. I don't see how anyone else other than you can weigh the pluses and minuses, ideally comparing these two amps in your system.
Unfortunately, I live in the Sierra Nevada Mountains near Yosemite Park. The closest high end audio stores are three hours away and they don't carry either Plinius or Pass Labs. To go to the greater Los Angeles area to hear these units would be over 8 hours away and LA is always a nightmare to travel in. So I'm hoping so of the audiophiles here can be helpful in my making a decision.
Have been in this hobby since 1957,and have owned countless gear.Had tubes when that was all there was.Have owned many Nelson Pass products from Threshold,and I have heard the X350.If you can afford the X350 -- BUY IT --.But b forwarned the Pass X350 is intolerant of weakness in the component chain and will expose those weakness's.I own Threshold Class A amps. They are rock solid in every respect.I have tried other preamps with the Threshold amps but always come back to the Threshold PreAmps,as the match to Threshold to Threshold is clearly superior.I would dare say the same could hold true for the current Pass Labs product. The X350 I listen to was using a Pass Labs preamp. If and when the Threshold gear gives up the ghost will be going to Pass Labs.Lets face it Nelson Pass had the ground breaking white paper on Cascode design in the late 70s and was awarded a patent for that design. He has been further refining that orginal design.Make no mistake Nelson Pass designs and products are timeless.I have wasted far too much time and money trying to find better product.
What about the SA250MkIII with siltech silver wire upgrade vs. an SA250MkIV? The MkIII can be had at bargain prices these days. Is the sonic difference between the two worth the difference? I know this is slightly off topic but I think those reading this thread would be interested in this issue.
You should check it out, but I believe the only difference between the MkIII and the Mk IV is the upgrade to Siltech silver wire.
Rayall: My understanding is that the MKiv has 4 separate toroidal transformers...and that the MKiii has one large one. In terms of sonics, many say that they are very similar...but some state that the MKiv was a significant improvement. The question much better is the IV if at all...and how much is that worth. This hobby is fun...but also can drive a person up the wall!!!!
I emailed support at Plinius to ask the very same question about the Mk. III vs. IV. I'll post the answer when I receive it.
Here the reply from Plinius support re: the difference between the SA250 Mk.III and IV.
The detailed changes in the SA250 are...

MKII And MKIII Started Approx April 1996 through April 1999
Serial number 7641 to 8931. Wired with the original Siltec cable. ALL MKIII wired with Generation 3 Siltec. U chassis design. Neither up-gradable to MKIV Single Transformer. Easily identified as not being a MKI by checking for 4 positions for ACS switch on the rear panel. Not easy to tell MKII and MKIII apart, only cables changed. MK IV Started June 1999 through today. Serial number 8931 onwards. Wired with Generation 3 Siltec. H chassis design. Quad Transformers. Easily identified as a MKIV by checking for 6 Countersunk screws on the rear panel or having a removable base. We believe that the MKIV was a step forward sonically. The power supply in particular seems to have added a degree of extra refinement and weight.