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

Ayre makes some very nice equipment too. All the brands mentioned are high quality so it will probably come down to personal taste, if you have the chance read all you can and give a listen. 

Of those, the one you must audition is the Ayre. It's got such a distinct sound you'll either be entranced by it, or scratch it off your list pretty quickly.

The A21 will run warm, if not hot. 

I like the AHB2 with speakers like yours because it's capable of the woofer control such speakers require for articulate bass. 

I owned the Parasound A21 for two years. For whatever reason, I never warmed up to it. It's good value for the money, but if you can stretch your budget there are better options out there. Of course, I'm mindful of the fact that it could have been a victim of synergy, or the lack thereof, with the rest of my system. But the good thing is that the resale on A21s is good. I sold it after two years without losing a dime.

In my opinion you've picked 4 amplifiers that run across a spectrum in terms of sound character. So I think it depends which direction you want to go, from your current amplifiers.

The AHB2 should run the coolest, due to its power efficiency.