My biases so far:

I have owned a Parasound HCA-2205A 5 channel amp (225 watts per) and I feel that the regular Parasound line is in the same league, or marginally better than the top channel Outlaws. No notable inprovement.

I have also owned in B&K 4330 (220 watts per) gear I feel that it too was on par with Outlaw higher end amps. No improvement!

The only amp I have previously owned that was significantly better than my 770 was The Parasound HCA-2200II. To me this was in a different league. As far as I understand, the new Halo A21 is the decendant of this amp. So far I am leaning in this direction.

Krell and Byston are top on the list too, but I do have not hands on experience here.