Bryston 4B3 vs Mac MC462 vs Pass Labs X250.8, OR... GRYPHON Diablo 300?

I have a Bryston 4B3 and McIntosh 452.
I spent 90% of my listening time using the 452.
Soundstage, imaging, musicality I think is
better with the 452. Bryston sounds somewhat dry.
Still a very good amp.

Wow!  It's certainly nice that you're in a position to consider such a wonderful selection of amps.
IMO, the Bryston and the McIntosh are at opposite sides of the spectrum. I agree with ajpk1971 that the Bryston is more dry sounding yet has a somewhat "in your face" presentation. The new McIntosh MC462 provides a sound that is on the warmer side, typical of McIntosh products, nevertheless full and robust. I understand the new model provides better bass than the outgoing 452. There's very little listener fatigue with the Mac. You can listen to it seemingly endlessly and the result is you tend to hear and be involved in the music rather than the amp itself. Personally, I'd put the Pass Labs unit in the bottom half of your list. Diablo 300 is an outstanding audiophile integrated. I would place it between the Bryston and Mac in terms of sound reproduction but it seems to give you the best of both worlds. My personal preference is to stay with separates in your price range but it's really hard to fault the Diablo's quality sound regardless. It's great that you have the ability to demo all of these, hopefully at the same time. Make sure to try different preamps with each of the amps. You'd be amazed how much difference that can make to the sound. With the Bryston and the Pass Labs, might I suggest you try a tube preamp? This could "tame" these units into something quite wonderous. Given your speakers, the Mac and Diablo should be really good matches. For your 803 D2s, the Bryston or Pass Labs with a tube pre could also be magical. Depends on your sonic preference though and how they work in your particular room. I certainly envy your choices.

Thanks for the input!  I will have a Mac tube pre-amp on hand for my demo as well as a Bryston Pre-amp, so those will be good to mix and match.

To be honest I’m not sure where I fit on the spectrum of engaging/detailed vs warm and non-fatiguing.  Although to me being able to listen to music hours on end might mean that the music isn’t quite as engaging.  If it’s a little more fatiguing I don’t mind if the music is more engaging....

For this reason the Gryphon may be the perfect balance between the 4B3 and Mac  for me.  However I’ve now seen a couple  posters have mentioned that the Gryphon can be a tad “dark” sounding.   While warmth is okay, to me dark is not a positive quality as it means a consistently lesser tonal response as the frequency goes from low to high.

I have a Pass 250.8 + SimAudio 740P preamp combo which I really like.  About a year ago I changed speakers (switched from Magico to Spendor) and the 250.8 was chosen as it mated very well with the Magicos (e.g. lots of power and a little warm sounding).   Over the last summer the heat the 250.8 puts out into my listening room (with no AC) was getting me down so I thought about swapping amps since the Spendor D9s I now own are easy to drive and not as lean sounding as the Magicos.   I figured I could maybe save some $$$ and not have to deal with the excessive heat the big Pass puts out.   The Bryston 4B3 was my first choice to try as it is very well reviewed, bulletproof, and I'm a previous Bryston owner of multiple pieces and a big fan.

I purchased a used 4B3 and dropped it into my home system after disconnecting the 250.8.   It was everything I read about, great bass, fast, smooth, very revealing.   Compared to the Pass I found it had a more forward and detailed sound, but it was still smooth and not bright.   Just a different perspective.   With my D9s I think it even had better bass then the 250.8.   Side to side soundstage was about the same. 

Where I found the 250.8 beat it to my ears and tastes was in the front to back soundstage and the feeling that the instruments and people were more real or 3D like in my listening room.   The 4B3 sounded like an amazing 2D ultra detailed recording, the 250.8 sounded like the real thing because of the extra space and depth in and around all of the sounds.    I ended up keeping the 250.8; in my system its output with good recordings just sound so much like live music (maybe a handful of rows back though).

Good luck with your search...