Personally, I think form factor, power, and price should be your main consideration. The A21 sounds fuller, AHB2 sounds cleaner (and I'm guessing it's the accurate one). The AHB2's highs are IMO cleaner than the A21's, but I think piano transients are better on the A21 than on the AHB2 and for that matter many other amps I've heard. But as Kal says, the differences are subtle -- I don't think I even noticed the difference in piano attacks until I did a level-matched A/B test.Thanks for the reply. Am considering the AHB2 and have heard good things about the A21 and wondered which amp you preferred and why.