Tboooe, both the Pass Labs X-350.5 and Classe Cam-400 are excellent amps that offer great build quality and wonderful sonics.
That being said, I have heard both in my system, and find the Pass Labs amps to offer a "musicality/natural" quality that I did not find in the Classe amps.
At this level of gear, it really comes down to personnal taste and system synergy. The only way to really decide is a home audition, you can keep asking for informed opinions, but your own ear's have to guide you.
Since, I use a Placette dual active line stage, I can't be informative regarding the Pass Labs pre-amps, it's been to long since I last had one in my system.
Historically, I had Aleph's 1 before the 350.5 in my system. When the X-series amps came out I auditioned a 350 and found it quite dynamic/detailed/transparent, but somewhat "cool" and less musical compared to the Aleph's, and therefore it was not my sonic cup of tea.
The 350.5 has all the virtues of the first generation X amps, but adds good taste of the "warmth/musicality" of the Aleph series.
I'm excited that later this week I'm going to be auditioning a pair of XA-100 monoblocks, to power my midrange and ribbon tweeters of my MG-20R's.
Like you, I have gained all the information I could by
talking to other fellow audiophiles regarding the difference between the .5 series and the XA series, but now I have to hear them in my system to really tell what the difference is and will I like the XA's sonics better then my 350.5's performance.
So, I think the time has come for you to set up some home auditions if possible, its both fun/exciting and at the same time a pain in the ass, but the only real way to tell. When I last did this with power amps I auditioned five different amps before I got what I was looking for.
Finally, for my ear's, with my gear, the Pass Labs X-350.5 is just a beautiful sounding piece, and somewhat of a bargain at its price. I believe you got to spend alot more, such as 600.5's or XA series amps, to get a better sounding amp and how much better is open to speculation.