How much are you looking to spend?

The Hifonics BX1500D will give you 500W at 4ohm. Not the full 600, but this amp has plenty of headroom if you ever upgrade in the future. It'll put out 1000W at 2ohms and 1500 at 1ohm. They're also relatively cheap at about $250.

Check out this page if you decide to rewire your subs...
http://www.jlaudio.com/tutorials/wiring/index.html