Mini Monitors under $500 used

ACI Sapphire III or Anniversary if you can find a good deal. I thought my old Sonus Faber Concertinos sounded good but the Sapphire III were much better.

Monitor Audio 90, $60.00.

Right now the Airmotiv monitors are on sale. You can get the Airmotiv4 or Airmotiv5 for under $500 and they have built-in biamplification. This is a high-tech composite slot-loaded woofer with a Heil-type folded ribbon tweeter.

There is also a pair of Quad 11 L2's on A-gon for $450/pair. I have no affiliation with the sale or seller. Those little Quads are fabulous and relentlessly engaging. Beautifully made, too.

Are the amps in the Airmotiv products actually good Hi-Fi amps? (Hi-Fi is my new old term for audiophile level) It's hard to believe they could be good, and the speaker drivers being good at the same time for, that price. I may just be so caught up in this hobby where money is spent like water, that I have lost all perspective.
I was told by a speaker pro that Monitor speakers were good and not expensive. I therefore believe you about the Monitor 90s, although he recommended a different model. Perhaps I shouldn't be so surprised.

For detail and soundstaging I think the Silverline Minuets would be a can't miss deal. I heard them at an audio show in NYC and they were really incredible in both those departments with mediocre upstream equipment. Best of luck.