Upgrade from Emotiva XPA1 to Bryston 28bsst2 or Pass X350.8

Without knowing the rest of your setup it's hard to say whether the amp changes is really a good value for your money.  Either amp may be better than the Emotiva, but possibly only a subtle upgrade within your system context.

I can't comment on either amp you're considering but I had a pair of the Emotivas on hand for a couple of weeks.  I directly compared them to a Theta Intrepid and a Cambridge Audio 840W.  Speakers were BG Corp 520dx or Meadowlark Audio Herons (hot rod version).  My cables are all DIY but they are quite good and all plugs on the power cords are either Oyaide 046 or IeGO's top 8095.  Sources included a Modwright Oppo BDP-83SE with tubed power supply or a Cambridge 851C both driving the amp(s) directly.  The Theta was run 4 channel to biamp each speaker. 

To make it brief, the Emotiva amps were clearly outclassed by both the Theta and the Cambridge.  The Emotiva bass was full and I could see some speakers sounding better, bass-wise, with the Emotiva over the Theta but, the Cambridge had a much tighter, more articulate bass range with greater impact.  No comparison there.  And that was the closest the Emotivas got to either of the other amps. 

The Emotivas were considerably less detailed than the others with less distinct image outlines and never created any sense of depth in my systems.  They were always 2-D and never palpable but that's not to say they weren't musical.  They were always pleasant to listen to but, compared to the Theta and the Cambridge, they weren't at all engaging as I was always aware that I was missing quite a bit.  Pulling the Emotivas from your system for the Pass should be a big upgrade if you've good enough cabling (junk cables are always a bottleneck in a potentially good system).  I haven't heard the Bryston you mentioned but wasn't impressed with the 4BST as it just sounded dry to me (but that's been many years ago). 

I’ve heard the Pass X250.8 in my system. It was a lovely, liquid sound, slightly midrange-forward to my ears and easy to listen to in the treble. I would have kept it, but I couldn’t tolerate (in a basement room) the 400W of heat it puts out at ldle. If anything, the X350.8 should sound even better, and run even hotter.

For years, I used a Bryston 14B SST, and I thought it was the best amp I’d had. I still miss it. Reviews of the 28B say it’s in an even higher class in the treble especially, and the bump from SST to SST2 is supposed to add a little more to that. Comments about a 4BST, though certainly offered in good faith, won’t reflect the sound of a 28B SST2.

So, I think you have named two great choices, and the only way to know which one is right for you is auditioning in your own system and room.

I would also suggest the Parasound Halo line, and for mono blocks, the JC1.  I went from an Emotiva XPA amp to a Halo A21, and everything got better.  The imaging, the bass, the vocals, the highs, it showed how entry level the Emotiva amps are.  

Bryson 28sst mono blocks excellent. Owned a 14bsst at one time and buddy just purchased new 14 b cubed and is very happy.