Pass labs upgrade path

I have the pass XA25 running with a Khozmo passive.  And upgraded resistors.  I think my system has a neutral sound.  I have horn speakers as well.  I won't be buying anything else.  Really good sound. 

@jjss49 Perhaps if I would have used the more "modern" CJ preamps like the ET series I would have a different opinion of how they sound with Pass amps. 

@marco1 

yes i will get around to trying it, likely this winter, but of late i have been pursuing the 'less is more' angle, using dacs that have good volume control schemes to directly drive power amps... 

I’m not sure that the middle end of the Pass line stages have ever gotten any attention till the XP 32 came along and that is second from the top. I have always been a big fan of pairing tube line stages with solid state monos. I’ve tried Mac 2500 with my Luxman M900u monos with my Wilson Sasha DAW’s and very unimpressive then moved to the Arc 5se still not where it’s at, although the bass response improved quite a bit but was still missing the musicality and soundstage. Finally I moved to the BAT VK53se and at least I was able to live with that for a few years. A major step up in depth and a flatter frequency response and far better than the Arc Ref 5se and Mac in my system anyway. 
   I finally came across a deal on a Pass XS, top of Pass’ linestage in their preamps. It’s a two box unit with power supply and control box. Each channel and the digital control circuits have their own isolated power supplies. All I can say is Bang!!! All of these other preamps  I’ve been messing around with for the last 10 years are ready for the pasture. 
 Pass XS spared no expense on building this preamp. Ceramic circuit boards with gold plating. Soundstage depth, clarity, resolution, bass but just enough.
   In the owners manual Nelson Pass admits that “if you’re on a restricted budget you might be wise to avoid borrowing one of these.” All I can say is all Pass Preamps are not created equal. 

I can't say much about your actual question, i.e., moving up to the .8 series.  However, I have the 250.8 and it is excellent. People say that they like the 250.8 better than the 350.8, but all I can say is that I am very happy with it.  I have also heard great comments about pairing the XP-22 with the amp, and you will probably get more bang for your buck upgrading the pre.  I went in a different direction . . . I went from running my MSB Discrete DAC direct to the 250.8 to using an ARC REF 6 pre.  To my ears, its a perfect combo with my Wilson Sabrina X's.

I have an XP-22 preamp and it is a very articulate and quiet preamp.  Also, it will drive longer interconnect lead-lengths in the case of having the preamp and source equipment a good distance from the amplifier(s).  Built like a tank like all of the Pass-Labs products.  I bought my daughter an XP-12 and had it for a few weeks prior to getting it to her and the XP-22 was a little quieter and perhaps slightly more micro-dynamic than the XP-12 but both are great preamps.  Good Luck !