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

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.  

Hi zwango!  Try the Starke AD4.320. I have two of them. Superb sound. WAY less than $2K. Never heard a better amp. I also have two of the Purifi Audio EVAL 1 packages. These are the best amps available. Great reviews. No kidding.Happy holidays.

For (nearfield) listening the ABH2 will be the best. And, the transparency will enhance the ’British roots’ of the Harbeths as pointed out above whilst keeping tight woofer control. Oh, be sure to get the inexpensive Benchmark Media cables especially if you are in a home office environment with I assume with computers and so on. Finally, it has the best volume control of the bunch - 0.5db increments. Makes a soundstage difference. BM has a 30 day money back trial and no-one sends them back.

Which preamp are you using? You can tailor the ABH2 to the source output. It runs cool and sounds like it is not there.

I just replaced a pair of Constellation Centaur Mono with the ABH2s bridged in mono.  They are fantastic, punching way above their weight and cost.  Extremely detailed and dynamic. Huge soundstage.   I’ll be using 4 of them and a Pass XVR 1 to biamp a pair of TAD R1s.  Will update as that project moves along.