I am in the minority in not liking the B22 sound as to me it is sterile and uninvolving unless matched with a more aggressive headphone like an RS1. Use it with a 701, 650 or any other smoother HP and it is too boring for me.

Perhaps that is just the boring headphones themselves coming through?

I think the heat issue of the B22 or Dynahi is a bit overblown. Compared to a GOOD stat amp, or a GOOD speaker amp, they're hardly space heaters. Relative to other headphone amps, yes, mabye so. I don't think either one runs so hot that you need to be overly concerned in your environment. Just make sure that whichever you build, you overbuild the heatsinking, and add a few vent holes to the case.

It's been a while since I compared AMB's Dynahi and B22. I do recall both sounding extremely close in overall performance, with the B22 leaning slightly softer, with the Dynahi having a bit more edge to the sound. I've also owned a Dynahi, which I can say nothing bad about, sound-wise. I really don't think you'd be disappointed with either, and the differences are much more subtle than those between headphones or sources.

ASR is selling a balanced M

Really hard choice. Having compared a 2-channel Beta22 (well, technically "half of a balanced" Beta22) with Sigma22 PSU to a Dynahi with Dynahi PSU, I'd say they're both damn good and you'll be happy with either. I thought the Dynahi was a little warmer and perhaps more involving to listen to while the Beta22 was gloriously neutral sounding, it was just fantastic and did everything well, though perhaps less "intimate" than the Dynahi. Do note though, that Dynahi is a heat-monster, felt like a little space heater on my desk.