I have never actually listened to either the Parasound Halo A21+ or the Bryston 4B3. I am trying to arrange that now.
I don’t have an informed opinion. At best I have uninformed concerns as follows:
The Parasound Halo A21+ is rated 500 wpc 4Ω (which I like) costs $2995, however THD is rated <.1% which does not impress me. The amp I am replacing has THD <0.005%
I also know that Parasound runs hot as hell at my listening levels.
The Halo JC1+ mono-blocks cost $8,495 each which at $17,000 for the pair puts them out of my price range. The Halo JC1 (no +) is an older and inferior to the Plus and still cost $5000 used. It does spec much better.
The Bryston 4B3 in contrast MSRP is $5695, almost double the Halo A21+ ; about 1/3 the MSRP of Halo JC1+. So one can not fairly compare three items of such vastly different costs. Suffice it to observe the Bryston spec much better than the A21+. And I can find 4 late model Bryston 4B3 on Audiogon right now.
The halo + items are newly introduced; I could only find those for sale at a dealer for full price.