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

Is size of concern?  Where will you place them?  Whats your source going to be?

Perhaps a powered monitor like the Tannoy Studio Gold  8. 

Is size of concern?
* Yes. It will be 3 feet away in a small room. Otherwise, the desk is reasonably big.
* Where will you place them? 
* On either side of double 24 inch monitors
Whats your source going to be?
* Computer audio

Tannoy is powered. Not a problem but since I have power, is there another Tannoy you like that is passive?

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!