These are not recommendations, just alerting you to the fact that 5 channel amps for $600 can be found, you just have to dig a little.
Parsound HCA855A (new) 85wpc x 5 for $595
Sherwood AM9080 (new) 120wpc x 5 for $599
If you can up your budget a little:
Chiro C500 THX Ultra (new) 110wpc x5...