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

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). 

I own nht C1's and also kef q150s.  The nht's can be found less expensive than the link provided, just do a quick search.

I prefer the Kef's personally, but ymmv.

They are quite different. NHT are sealed, Kef are rear ported. Kef are quite a bit larger footprint. 
NHT are more forward sounding. Kef have more bass. 
NHT look nicer, nice piano black finish. Kef wood trim looks pretty cheap. 
But to me the Kef sound better playing from Bluesound Powernode 2. 

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.

Sorry never heard R100s so couldn’t say.

I’d assume probably little better but would have to hear both. R100 higher series but also older series I think right? I’m no Kef expert, just grabbed the q150s when they were on sale half price a month ago to get some more variety and was really impressed (admittedly probably wasn’t expecting too much but they were better than expected). 

I think part of it is I’m pretty underwhelmed with my Spendor SA1. They aren’t worth their msrp.

If I include my D1 (3.2k msrp) in the mix as a true reference bookshelf speaker imo (they are fantastic), and rate the D1 as 100, then the SA1 drops to 77, the Kef q150 a 74 on this scale, the NHT C1 a 67, and Absolute Zero a 42.
Just my opinion and damn I am really rambling on here but maybe this helpful for some or maybe n........

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. 

I agree on those x14. Before I got the Kefs I was thinking possibly new x14s at $800 or 900 whatever they go for now but didn’t want to spend that much. Then the Kefs went on sale at 300. But the Dyns are better for sure IMO.

I also think those x14 are better than anything else around 500. But it’s all subjective. They would be for me. Ymmv.

Reason I’m so sure for me is when I got my D7 in main system I auditioned two dozen speakers over 6 months before settling on the D7. If my budget had been 5k (which it started at) I would have gone with Dyn Excite (whatever the 5k tower is, forget the number now, maybe x38 or x40). But once I heard D7 I knew I couldn’t keep budget at 5 k and would have to raise it a bit.

Dynaudio typically very hard to drive but that one x14 model isn’t as bad as most other Dyn bookshelf if you look at impedance and sensitivity and read the reviews.

then there are the x14a which are powered speakers but new they and x14 are outside of OP stated budget

Regarding SA1 that someone asked about, they have old tweeter instead of the new one that comes on D series. So they sound like classic Spendor which many love. For me it’s a bit too laid back. Some classic Spendor fans think the D series tweeter too forward but perfect for me. All subjective.

Incidentally the reason I ended up with so many Spendor and NHT bookshelves (I also have Superone 2.1 but never tried them for music, only HT) is they are some of only sealed bookshelves out there and three of my applications really work best with sealed speakers (one inside a cabinet, one on table near wall, third for rear HT surrounds mounted to wall).

I wouldn’t want damaged ones though.
I think that’d be the play for me if I was in market for $500 used bookshelves. Wait for a clean pair of used Dyn x14 at 500ish. They sell new for 800 or 900 for last 6 months at least so eventually should be some used for 500.

The one I’m thinking may eventually replace my Absolute Zeros is Triangle Borrea BR02. Why? Something different, horn tweeter, great reviews.Or maybe Monitor Audio Bronze 2 now on close out?

But I’m already a barely recovering bookshelfaholic as this thread makes clear so I gotta abstain for awhi.....