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

Hilde45, at 3 feet I’d look for concentric designs.  Something with the tweeter in the middle of the bass driver.  The Tannoys are a good suggestion but at $700 are out of your budget.  
I think you have the Adcom 535 correct?  I’ve owned the 535, 545, and 555.  The 535 is the sweetest, least harsh sounding of them all!  Mine ended up in the trash can because I accidentally touch the speaker wire leads together while the amp was on.  Be careful and never do that as there is no built in protection!

So, with that all in mind, a pair of Kef 100’s might work well being that they are front ported and ghat the driver sits high up in the cabinet so on a desk they’d be close to ear level.  You can find them used in the $250 range + $50 shipping.  Another option would be the Kef X300A for $399 + $50 shipping.  Theres a white pair for sale here on the Gon right now.  Nice thing about the X300A Is that they are active and have an onboard dac so the set-up would have a nice clean look on your desk!

JBL LSR 308 or 306 MkII for powered and balanced
or if you can find them JBL 530

Another option...

https://tmraudio.com/speakers/bookshelf-speakers-monitors/elac-uni-fi-ub5-concentric-bookshelf-speak...

I picked up new Kef Q150s last month when they were on sale half price for $300.  Some dealers may still have some at that price. 
They are very good. I rotate three sets of bookshelves at my office for variety. $2.2k msrp Spendor SA1’s, $600 msrp Kefs, and $300 msrp NHTs.   
The Spendors are the best, but not that much better than the Kefs, which themselves are much better than the NHTs. I’d say the Kef’s give me 95+ percent of what the Spendors do at 28% of the cost (14% of cost if you can still get them half off).