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

@canibefrank -- it's great to talk about options in audio when the constraints and functions are fairly defined.

@superultramega

Here’s an indication about size of speakers and distance from wall for ports —

Here’s an example of TOO LARGE:
PARADIGM MINI H 13.4 x W 8.3 x D 11.4

Here are examples of GOOD SIZE:

PINNACLE speaker dimensions:11.37 H x 6.75 W x 6.5 D

PARADIGM ATOM H 10.6 x W 6.7 x D 8.7

I think front ports would be helpful. I find it hard to get them more than 8” from their walls.

I paid $389 for my C1 pair new on Amazon about a year ago, FYI. Sealed is nice for desk and with only 8” behind. They are currently in my main HT system, which maybe could be considered a promotion from backup duty in my third music system, where the Kefs are (or maybe a lateral move, haha). If your amp is neutral or laid back, the C1’s should be fine. If your amp is forward, then maybe not.
The Kef q150, while rear ported, do include port bungs.

I’ll give a little more feedback on the Kef q150s. 
I am a Spendor fanboy. Have D7’s in my main system, D1’s in my secondary system, and SA1’s as the main speakers in my 3rd system, with the Kefs as the backups that I rotate thru (along with some NHT Absolute Zeros -ya, I gotta problem, but don’t judge haha). 
Anyway, the SA1’s have $2.2k msrp, the Kefs 600 msrp, and the Absolute Zeros 300 msrp. 
The SA1’s are the best of the three, but it is sure no landslide. 
For example, if I say the SA1’s are a 100 on a normalized scale of sound quality, I’d rate the Kefs a 95 and the Absolute Zeros a 60. (for me the C1’s maybe an 85, but if you like clarity and forward sounding speakers more than I do, you might rank them higher - I do not typically prefer forward sounding speakers). 
So at about one quarter the cost of the SA1’s, the q150s punch outside their weight class IMO. Ymmv. 

@kren0006,how do you think the Q150’s compare to the R100’s?