Best speakers under $500? Fire away!

B&W Matrix 805 used

Phenomenal imaging, very balanced and nuanced midrange, and decent bass for their size.

Used is about $500, and will retain that value for some time. Remarkably hard to beat for $500

Depends a lot on the type of music you listen to.  Your 3020 certainly images well ..... so used Thiel 1.5's or better yet 1.6's (much rarer) might fit if you are interested in classical or jazz or singer-songwriters.  Not the best for rock, though.

you can have my Infinity IL40s for $250...great home theater speakers.

I like the Dynaco A-25 with the NAD 3020. 
I think on par with the AR-3 for much less $. 
Not super detailed or extended highs, but very natural, smooth in ways modern stuff isn't. 
The A-25 is really an early B & O,  back when they made good sounding stuff. Easy to drive and I still think what 200 for a nice pair. 
If you like em, you can upgrade to a nice vintage tube amp for the leftover dough and use the 3020 phono stage. Exactly the path I had my friend take and it sounded very good. Upgrade was to Eico HF-86.