Best small, desktop speakers, $500 or under (new or used)

I have to add -- reading the reviews of the AudioEngine and thinking more about some suggestions here to clean up the desk area -- the Audioengines are enticing. I could be persuaded to give that Adcom stuff to my kid...

For office desktop I would go powered speakers to clean up the area, less wires as well. What are you using for a source? Some like these Fluance can connect to your computer with USB. 

https://www.fluance.com/powered-2-0-bluetooth-active-6-inch-bookshelf-speakers-white-walnut#!prettyP...

Get a pair of Google Home Max speakers. They can be paired wirelessly and they sound awesome. You can disable the mic for the Google assistant and can cast music directly or use it as a Roon endpoint. Aux in is available as well.

One thing these have is the ability to tune themselves depending on the environment they are placed. All for about about $400 a pair!

I use a pair of these on my desk everyday and I am constantly reminded of foolish my decision was on getting the LS50W.

You can easily build "the best small speaker I've ever heard" according to one Stereophile reviewer. It uses the SEAS tiny L12RE-XFC co-ax driver.
I built the cabinet recommended by SEAS and put the speaker into it. The speaker itself was $165 or so each. I bought four. I used a sealed cabinet made of counter top material for one pair and the recommended cabinet design for the other pair. Not much difference in sound. The sealed cabinet is very easy to build; use the same outer dimensions as the recommended cabinet. I've used engineered quartz (recommended) and granite (it can crumble in your hands and is NOT air tight) for cabinets.

Madisound sells the speaker and M'sounds own fancy crossover (mount externally). Broken counter top material is cheap and the shop will cut it for you. The cabinet will be acoustically inert. Make the front piece from some nice wood and glue it on with the speaker mounted on it. MDF on the rear, cut to fit flush with the counter top sides and bottom. You're out for less than $600 per pair with nice terminals. A subwoofer never hurts, but they will surprise you running "barefoot."

If you aren’t rocking the office walls down, the A2+ should work just fine. They are tiny little guys so the size may be appealing and the red ones are pretty neat looking. Theyhave a nice tone but are rolled off on the highs. 
The Adam T5V is twice the speaker, but twice the price! You could buy either new and if you don’t like them return them! The adams are much larger.

Adam T5V
11x7x11
45hz-25khz
5”woofer


Audioengine A2+
4x6x5.2
65hz-22khz
2.75” woofer

You’ll want the angles stands for the A2+ and those are around $35 so take that into consideration!

Think of the A2 as a pretty good sounding computer speaker. Like those plasticky altec lansing or cambridge audio computer speakers that you would see pretty much anywhere. Like the ones you see on craigslist for $10. The A2+ competes with those but the A2+ is prettier and has more solid bass and better tone.

The Adam T5V is a speaker that someone would use next to a computer for music production. The Adam T5V is going to have a much more extended low end and and high end and the AMT tweeter is going to offer much more detail. The Adams are real speakers.

I’m not bashing the A2+; I have the B2 which is a pair of A2+’s connected together without the dac. I paid $125 for one from Audioengine and it was refurbished.
If I were to buy the A2+ And the stands that would be $300 ish. Since the Adam T5V is $400 and within your budget, I’d go for the T5V for sure. The T5V will walk all over the A2+ soundwise.
The A2+ has a dac you won’t use, a conventional crossover and a single A/b amp outputting 30watts per channel.

The Adam T5V doesn’t have a dac, it’s crossover is handled via dsp (done in the digital domain) and uses 2 class D amps per speaker For a total of 4 amplifiers outputting 50 watts to the woofer and 20 watts to the AMT tweeter; 70 watts per channel.
The A2 is a cute lifestyle product that sounds decent. The Adam A5V is a decent looking speaker that sounds really good.
You’ve got to weigh which is more important for you in your application! 100% looks with 50% sound or 100% sound with 50% looks 😁