I am not sure if linking another web page is welcome overhere, in terms of a-gon forum policy... If not, I apologize. If so, I would recommend this page:
http://www.stereophile.com/content/bryston-7b-sstsup2sup-monoblock-power-amplifier-page-5
According to Mickey Fremer's opinion, the Parasound top monoblocs Halo JC-1 (400W) pretty clearly outperformed similarly priced Bryston 7BSST monos (600W). The next step-down in both brand lines are exactly the Halo A21 and 4BSST respectively... so, a possible outcome of similar comparison may be easily predictable, at least for ears similar to mr Fremer ones...
I would not vote, of course, since I have never heard any Bryston in my system. But as a former owner of the A21 I can say this amp is one of my most inspiring experience and true entrance-gate in (so-called) hi-end, for minor money. This particular piece of Halo A-21 ended at my friend's place, making him happy last 5 years or so.
I have extensive experience with the A21's bigger brother, the highly reputable Halo JC-1 monoblocs, comparing them many times to Krell-Evo-400 and Evo-400e monos. I can say Halos have not any reason to be ashamed in all those showdown parties with double-priced Krells. Actually, the outcome had turned vice versa sometimes, depending of preamp, speakers, music etc. In terms of final verdict, I would say Krells emerged as a bit more audiophilic, more neutral, sober amps, while Parasound Halos appeared mesmerizing with euphonia, sweet sustain-prone highs and delicious, meaty midband.
As far I could recall the sonic nature of smaller Halo A21, this is the very same type of sound. Warmest recommendation.