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

I think the best sounding product between you options is the Ayre V5xe.  I have bought equipment that was driven by that amp.  It is built like a tank and Ayre gear sound incredible.  It is best when in a fully balanced system but is tremendous in any scenario.  This assumes the amp is in great shape.  If the amp is not in great shape, I would lean toward Bryston.  

I really appreciate all you friends' input and discussion. These are all great amps and that was why I could not make up my mind. The Arye V5xe is local so I will go pick it up for an audition with the Harbeths. This system is mostly put on low volume with classical music. But occasionally I turn the volume up and concentrate on the music for a while. I'd like to have good openness (airy), separation (resolving) and positions (imaging) of the different instruments, etc. Right now the Harbeths are connected to a couple of Music Fidelity M1's and a PS Audio Perfect Wave DAC (no Preamp).Happy Holidays!

Ayre

I think it all depends upon whether you like a warm sound or a brighter sound and how they match with your speakers.  I would certainly try to audition them if you can side by side or at least sit down for a couple of hours to hear what sound most pleasing.  What will you like best when hearing them for two hours.  This group should be a big help.  You are lucky to have them.  This is the right place to start.  Ask them about what to be prepared to listen for.

it would help to know what preamp you will be using so we can recommend an amp with good synergy.  your speakers are already on the warm side of neutral.  
i have the Parasound new model a21+ and it sounds excellent driving my more neutral Harbeths with a tube preamp.  
It does run a little warm but not room heating warm, however its size and power seem over doing it for an office.  
if your preamp is neutral to slightly warm the Benchmark should be perfect provided your pre has balanced outputs.