Best interconnects used for around $500 +?

Mrotino describes the Au24se well. I use them myself pretty much throughout my system. The only thing I'd add is that you could consider also trying Audience's Ohno ICs, which come surprisingly close to the Au24 but at a fraction of the price. 

Purist Audio Design Musaeus :-)

Forget about the AU24SE.  Should be considered the Audience SX series for ultimate musical sound just for a little more money but well worth it!  I had them both in my system and the AU24SE is not even close! 

The Audience SX delivers performance that other cable manufacturers charge 3-4X the price. 

Check out the Cerious Technology cable thread.

I've recently replaced my FMS Nexus IC's with Von Gaylord 7000 ICs. If you can find a used pair of the FMS Nexus cables, you'll be more than happy. They were really expensive when new, but can be picked up within your price range used.