@jfrmusic, I purchased the 4B3 and AHB2 while I owned the 250.8 so I could directly compare them with the big Pass amp. I ended up slightly preferring Pass amp over the other two, but it was really just pros/cons of each and ultimately with my system I slightly preferred the 250.8 so I kept it. At that time I likely would have purchased the 4B3 if I was going to pick one of the two. The AHB2 was and has been the clearest (like a super clean window) amp I've owned allowing me to hear into the recording the best. I loved its size and lack of heat. But, I personally found its presentation a little different than what I was ultimately looking for, more like you are walking in and around the images, like I'm in the studio with the band. I also found its texture and sound stage depth to be lean compared to the other amps on my list, and most specifically when compared to the Pass amps. I found the AHB2 a great tool for analyzing the recording, but found it harder to picture me in the audience enjoying the show, which is what I'm looking for.
The AtmaSphere GaNFET monos caught my eye when I heard Ralph was working on them since he has made a living sounding tube equipment and to me the Pass amps have what sounds right to me about music from tube equipment. I dropped in the Class D monos and they instantly sounded like the Pass amps I've owned, zero fatigue, a hint of warmth, great soundstage etc. The Class D monos give up a little sound stage depth and midrange texture (at least compared to the XA30.8) in my system but have the similar Pass sound with a blacker background, tighter bass, and a crazy good lateral soundstage. The one area where I think they differ the most with the Pass amps I've had is I think their high fequencies are a little more recessed,with less sparkle. Their HF output is definitely not in your face at all, but maybe a little too much so, depending on your system. I purchased the 250.8 to drive Magico S1s and Spendor D9s, both of which are not overly forgiving on the top end. If your system is already darker or recessed in the high frequencies, you might find the monos to be lacking too much HF detail. Overall though, I'm very happy with them as Pass 'replacements', and they ship in one box and literally generate no heat!
If I go back to class A or AB again, at this stage in my life it will likely be with a less than 50 to 60 pound requirement. The Coda 5.5 at 45 pounds would be an example of an amplifier I would love to try next and compare with my Class D monos, and the Hypex Nilai monos if sticking with Class D.