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

My two cents and ears, for budget choices:
First choices:
Triangle Zetas for 200 on Amazon...detailed, a bit forward, musical...I have these. Someone once stated they sounded similar too his Spendors.Cambridge SX50 (200) or Minx XL (300) more laid back, "soothing" in the words of one reviewer.

Second choices:Others Infinity R152 (200), Dali Spektor 2 (now over 300). R152 sounds similar to Kef Q100s and I liked them better, a bit more detailed than Zetas but more clinical IMO.

All should be easy for any amp from a cheaper class d, ab, etc to more expensive options which bring out the best in them.

Would not recommend Paradigm Atoms kind of dull and awful for in room stereo, Ascend (too homogenized, everything sounds same), Polk or Klipsch (cheaper models are banchees). 

If size doesn’t matter and used OK...there’s a diversity from which to choose.

I've chased this for years. I decided internal amplification for computer/desktop was the way to go, then improve the source: high res music files, good soundcard or DAC, Amarra, etc. I started early with new Altec Lansing ACS48s - loved them. Then B&W MM-1: excellent and fun but, like Audioengine and my Adam Artist 5s, all sounded accurate but slightly processed to these ears. A pair of used Swan M200 Mk II arrived with damaged internal amps but sound terrific with external amplification. I love our sublime Totem Arros and other small Totem designs have that magic but I'm not tempted by the sound of the self-powered Kin Play. My keepers: 1) Decware DM944s (now DM945) - I used then nearfield for awhile but they deserve more room – and tubes. This brings me to 2) Vanatoo Transparent Ones - the knock-out punch. Music leaps and thunders out of them. Jaw-dropping yet very musical. Miraculous. The Zeros are smaller, and perhaps cooler design-wise, and equally astonishing for their size - I've heard them at length and in conversation with one of the owners. For context, I always buy used; myroom-sized systems have included Klipsch Quartets and Klipschorns; Merlin, Mirage, Allison, Canton. Omega 7 XRS Alnico, Tekton OB4.5. My keepers are all full-throated, balanced, transparent and tonally rich to my ears: Klipsch Epic CF-3, Decware DM944s, Totem Arros – and for desktop use, the Vanatoo Transparent Ones.

+1 For Audioengine A2+ with their DAC. That is my office system. 

ADS l300s

Another vote for Vanatoo Transparent Zero, incredible speakers from a very responsive company. You must use their interconnect between speakers, it is NOT a regular ethernet cable. Excellent sound from a tiny set of speakers.