Any advice? Parasound A21, Ayre V5xe, Benchmark AHB2, or Bryston 4B3?

+2 for Ayre.
B

I tested out the Benchmark and was somewhat disappointed by the sound, the hype was so good. Indeed it may have good bass, but lacked a precision to resolve high frequency detail, imagery and delicate subtleties that give the sense of air and space, and most important the subtle artistic feelings.
Ayre and Bryston would be better. Though both need good airflow around them. 
Funnily enough I was using Harbeths in the amp test. The Benchmark sounded good in terms of amp / speaker pairing.

Hegel (great value) and Gryphon (great – period!) both work superbly with Harbeth. 

I have a pair of Benchmark amps as mono blocks. They are Unbelievably Good. So much better than the PS Audio amp I upgraded from.

I have two A21’s powering A pair of Harbeth SHL5+ and although it sounds lovely, both bi amp and Bieber I feel the Ayre is a better match, only because of the bass extension with the ayre. I do have to turn one A21 down very slightly for the super tweeter though. But for the money you cannot go wrong with parasound