Best interconnects used for around $500 +?

From my experience, I would highly recommend you try out either used Tara labs air 1's, or Zu audio mission (new) or even better Zu audio event (if you can find used) which should all be within your budget and provide an amazing value for money. I have yet to hear IC in that price range that even come close to these. But, there are so many different cables out there that I never heard... :-)

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.