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

ayre and hegel

deep black silence
timbral accuracy
smoothness/warmth/seductive
effortless drive and foundation
detail presented in an easeful flow, not unto itself

I am not familiar with Harbeths. But on some quick research, I think these are 6ohm speakers with 86dB sensitivity. If this is correct, long story short, you may be happiest with the AHB2s. They have a lot of features. You can run them as stereo amps or bridge them for mono use. They have adjustable gain and they have Neutrik NL4 connections. If you are interested in that Zu Audio makes nice speaker cables with NL4 terminations. These are quiet amps with low distortion. I own a pair of these and love them. I have also owned the Bryston 4B3s. They have more power. They can also be configured to run as bridged mono and are built like tanks with a 20 year warranty from date of manufacturing. It you needed or wanted the most power the Bryston is a good pick. But in an office with bookshelf speakers I think you will get the most clarity from the AHB2s with enough power and headroom for an office setup. 

Just a small addition to kingbarbuda's post: Benchmark also makes speaker cables with NL2 and NL4 terminations.  See their website.

...........a precision to resolve high frequency detail, imagery and delicate subtleties that give the sense of air and space..........

That's exactly what I appreciate about the AHB2!

Check out Hegel. They're coming up somewhat regularly on ebay and here for ~25 to 40% off list