I’m actually a fan of pass labs x series. They are very good solid state, without pretending to be anything else. (Of course, you are paying a premium for the pass brand.) I have heard the XS models only at shows, so I don't have an opinion on the m. However, I am not a fan of the XA amps. Because I own tube amps, to my ears XA don't fool me into thinking I am listening to tubes and instead sound like very badly colored solid state, an egregious offence to my senses. It’s kind of like you are dating a girl who dies her hair blond. But instead of getting her hair done once a month, she has missed her hair colorist appointment for 3 months. Nobody is fooled by those long black roots.
I strongly disagree with your remarks. I have tube gear myself (see my signature), and just like my amps the Pass XA 160.5 amps, as I heard them in Peter A.'s system, sound neither like tube amps nor like typical solid state with its slightly electronic sounding colorations. I cannot see how the Pass amps are badly colored as I hear a similar tonal balance between my amps and Peter's, and my amps have a very similar balance as the Spectral DMA-260 that have a reputation to be very neutral and that I have heard in my system, thus in direct comparison to my amps.
Of course, one caveat: I haven't heard the Pass amps in comparison with other amps in Peter's own system, which would allow me to circumvent the extrapolations made above. Yet if the amps were colored, then they would do a mighty good trick in neutralizing other putative colorations in Peter's system (unlikely; the more straightforward assumption is that all the components in his system are pretty neutral). Also, if the amps were colored, they hardly could reproduce human voices with the stunning, jaw-dropping believability that I hear in Peter's system (from the very best recordings/pressings, that is).
As my tube amps, the Pass amps excel in micro-dynamics -- very impressive for solid state amps.