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

Well, if you can afford it, that Bryston is one awesome product!   I am not familiar with the Benchmark; the Parasound is also good, being a John Curl design.   If it was my money, I would opt for one of those two.

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.

+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

If you share what you’re looking to improve upon and what kind of sound you’re looking for you’ll get more detailed and helpful responses here. 

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

we just purchased a hilo by parasound and love the unit.  you should audition what you like at the $2500 to $3500 price point but would not go wrong with Parasound in my opinion.

I have an all Ayre sysrem... all of their components sound best when you use them in balanced mode. ..never had any issues with any of their stuff.  Recommended

I have owned an A21 since January and only heard it through my Martin Logan speakers, so cannot compare to other amps/speakers as others here can.  I can say that my amp does not run hot . . slightly warm only . . but I have it open to room air and would recommend that.  

For that price range, Parasound has an updated model with more power, the A21+, which might be a better comparison to the other models you are looking at.  Normally $3,000, I’ve seen the A21+ on sale recently for about $2,500.

AHB2 has the most resolving power of the amps mentioned.. can’t believe someone stated otherwise. 

my guess, just an educated guess, is that someone who has chosen harbeth compact 7’s are not really into having resolving power at the expense of musicality and coherence

but op, you have the input you seek, benchmark certainly makes a good amp, and has its supporters

good luck making a good choice

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.  

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.  

Ayre or Bryston. 
 Happy hunting.

I have owned one and then two of the Bryston 4B3s. They are wonderful amps with lots of power. But for an office they are big. If you said you were looking to drive Maggies, these would be my top recommendation. But I also currently own a pair of Benchmark AHB2s. Very transparent. Very cool running. More than enough power. You can get nice speaker cables with Neutrik connections for them from Zu Audio either on their eBay store or their website. Those would be my top recommendation for the Harbeths. If you are looking for a preamp consider the Benchmark preamp. Small and transparent. Goes great with the AHB2(s) that will bring you clean SQ (that also measure great) in smaller form factors. Enjoy!

Another +vote for AYRE.

Happy Listening!