Hi @english210
Noble quest--complete sound, that is. I applaud you. I can only speak to my experiences with Pass equipment. I have had many of the AB "X" series amps with, mostly, Dynaudio speakers. The Dyns have the hallmark of smooth and natural with awesome imaging potential, but not the worlds most "punch you in the head" kind of dynamics other high end speakers might offer. Long term listening pleasure is what I admire.
Pass Labs have been wonderful on the various Dyn models i’ve owned over the years. The only XA amp I owned, was the XA30.5--I loved every second of music sessions with that. Never played a sour note. Just incredible.
The answer is system dependent; currently I am using Luxman amplification (pre and power) and i’ve been tuning the system to optimize this gear. EVERYTHING matters, and it’s not simple enough to just swap amps and hope for the best. Sure you can do that but to really find the potential it takes patience with trial and error of other supporting pieces. I am still learning this lesson today. Power cables on the amp alone can totally make or break the entire result. I am a trained EE and active in electronic design. I gave up trying to figure out how/why PCs make the kind of difference they do. I use my ears to measure and the differences can be startling.
A few fellow goner’s recommended Triode Wire Labs, for instance, and I am currently evaluating the Obsession PC. It has transformed the amp in ways I never thought possible.
Balance
If your speakers have metal dome tweeters and are super revealing, (and maybe like 88-90 dB sensitive ) and your room is about 12x20x8, and you don’t listen at ear splitting levels all the time, I think a Pass XA30.8 would work wonders.
If your speakers are soft dome, lower in efficiency, you might consider an X150.8/250.8...
If you have some form of high frequency tinnitus or irritation to harsh sounds you can’t go wrong with Pass (and Luxman, TBH, both are awesome in their unique strengths);
Your source cabling and component set matter greatly in this too...
You can have too much of a good thing-- tubey preamp, XA "type" power amp, warmish cables, warmish front end, smooth speakers (Harbeth/Spendor/Dynaudio) and find yourself swimming in a sea of pleasant mush...will not offend your ears in ANY way but quite boorish perhaps with a homogenized sound.
Again, good luck on your quest!!
-Dave