Budget friendly receiver choice

I want to stay with 5.1 (or better, don't see the need/benefit vs trouble of 7.2 etc) but also use the system for audio a lot. 

I have a dedicated 18" Dayton 18-22 sub (sealed enclosure)  with the excellent amp from the Elemental Design A7. Front tower speakers (various, experimenting, changing, still not found the final one). 

On my wish list:

- DSP

- pre out (so I can use other amps I have)

- nice: use build in amps for bi amping (Anthem MRX 710 i read offers that option0

- stable (to drive critical speakers if need be)

- low price certainly under $1,000 , $600 preferred (so used obviously)

- probably should be able to feed 4k TV (Vizio M75 currently)

- ideal (but not done anymore): pre/main amp split (remember these small metal U shaped bridges that good full amps had?) 

I mentioned the Anthem MRX 710 because I just saw one offered locally (without the software/mic). To stay under $600, I likely need to make more compromises. 

Suggestions (full make/model so I can look up prices and reviews)?