Hi Dave,
Greetings from Singapore (I live and work here, but I'm from London). I thought maybe I could assist in giving you a few pointers as to what differences I've come across between Accuphase class A and A/B. Please accept that my ears are my ears and maybe good, bad, old....take what I say with a pinch of salt. Over the last 2.5 years, I've heard Accuphase quite extensively (the Accuphase master importer and dealer is 5mins from my house here in Singapore and my wife is Japanese and we go to Tokyo 2 to 3 times a year where I listen to Accuphase setups there as well).
The Class A amps have absolutely wonderful midrange texture and a detailed nuance with a relaxed sound. As output is at full power all the time being class A, it seems things such as bass sound full even at low volume. This describes the sound of both Accuphase A-65 and A-46. I've heard both of these with a home demo with my Harbeth M40.1. My Harbeth's Dave are a different beast to the JBL's (wonderful speakers those 4700 btw) being only around 84db efficient, but they have a really easy impedance never falling below 4.5ohms and staying mostly between 5 to 8. So an inefficient speaker, but an easy load.
I found listening to the A-46 underpowered for orchestral symphonies, but the A-65 seemed to have ample reserve, though I found the bass wasn't as whip crack fast as I hoped. However, I've heard the A-65 with more efficient speakers with very braced cabinets like Vivid audio B1 (compared to Harbeth's traditional BBC lossy cabinets) and the bass sounded solid (so maybe the Harbeth doesn't get on totally with class A, don't know)
I then audition the P-6100 and with my M40.1. For me, this had that whip crack solid bass control and was very vivid. The Images were sharp and more carved out compared to the A-65, but the A-65 had a certain texture with acoustic instruments and voice. Best example I can give is with P-6100 you saw the clear outlines of a woman's mouth while she sang, where as with A-65 you could almost sense the texture of here tongue around the mic where as the outline of her mouth wasn't as sharply visible like the P-6100. But the P-6100 was very vivid, had more shimmer to the sound, whereas the A-65 was texture, nuance and relaxed. The P-6100 does sound a little faster. But it was in the bass that the P-6100 won me over. Absolute solidity with clear bass outline and rhythm. Not fat. Not hard. Had warmth, but was solid and fast well. Real world bass.
I heard the grand daddy A200 in Japan with the JBL K2 S9800 and they seemed to have the best of both worlds, but at a price..... I get every quarter the latest issue of Stereo Sound (Japans High-end Journal) and in the latest issue an owner of the JBL 9500 (Japan's favorite JBL speaker...ever!) a feature on customer setup at home shows him using a P-6100.
Phew...hope this helps, but as I said earlier, this is with my ears.
Regards
tmokbel