Pass Labs Power amps choices

I am selling my Krell EVO components and moving over to Pass labs.
I just snagged a very nice X-1 for a song and now need a power amp to move my Revel Ultima Salon 2 speakers.
So my thoughts are either 250 or 350 stereo amps. Question much different/better are the .5 and .8 varieties over the older x-250 or x-350 amps?
Many thanks.
I'm thinking that although the 250.8 would be a nice upgrade over the 250.5 that it would not be worth that type of money to do the upgrade. I had my 250.5 repaired a few years ago and it is working nicely. It would take me several years to pay back $10,000 so I will just enjoy my 250.5 as it is. My pre is an X1 which matches nicely to my amp and my speakers are Hales Transcendence 5"s which are nice. I just recently upgraded all my cabling from Cardas Golden Reference to Audio Art Statement interconnects, speaker cables and power cables. The upgrade was nice and I am very pleased with the results. I also upgraded my PS Audio Perfectwave MK II dac to a PS Audio Directstream with Bridge II. I am definitely enjoying my system in a way I never enjoyed it before. The Audio Art Statement cables were a huge upgrade over the Cardas Golden Reference and the power cables are much nicer than the previous cables I had. To be truthful I'm not that itchy to upgrade my 250.5 at this point. I believe the total cable upgrade throughout my system top to bottom was an upgrade I do not regret.
As I understand it the .8’s have a slightly more muscular output with a more tailored harmonic content, on the other hand the .5’s are more harmonically neutral (than the .8's). I believe Pass Labs will still offer .5’s on request.
Ya that is what I heard from many people that compared X to XA that XA is better in most respects and the build quality is closer to XS.
But X may be useful when more power is needed and maybe better with bass control and dynamics (Kent and Desmond mentioned this).