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

Nobody is going to suggest a pair NHT C1 ?

https://www.nhthifi.com/products/16548-c-1-bookshelf-speaker

Now Hear This being one of those long standing, good sounding speakers brands that's quite affordable. Easy to drive, well made and doesn't try to lend 'it's own' sound.

I'd second the Paradigm ATOMs that others have suggested. Still pretty hard to improve on for the size and price.

The powered Adams are for tracking / mixing and while they will reproduce a dynamic, detailed and full scale range of what's been recorded they're meant to be impartial and they are. No imaging what so ever, that's for home audio speakers. I use their big brothers at work in the studio and they're great but I'd hate to have to listen to them for long at home. IMHO

The NHT sounds interesting. Their website suggests 100 wpc for amps and the speakers are 85 db. I have a 60 w amp but won’t be listening that loudly...hmmm. I have heard and like the Atoms. 

In both cases, these choices don’t seem all that much better than my Pinnacles. I was hoping for a next level speaker. Perhaps too much to ask. 

Hey, maybe you should keep the Pinnacles. You seem to like them. 

The pinnacle speakers are disappointing but they are not so terrible that I might not keep them for home theater surround. They are decent but not great. I am hoping that for focused and near-field listening I can do better with a newer speaker as described in the original post — but I don’t think I can do better if the replacement speaker is at or below the price-class of the pinnacle speakers. That is why I keep returning to them as a metric.