Polk,
I think you may wish to try the Onkyo TX-SR503 with either the M60 or M80 first, before you bother to look at switching out to a more powerful amp/receiver. There are a number of forum members here that successfully use Onkyo receivers to power both the M60 and M80 model speakers. Quite a few receiver manufacturers state 8ohm only as sort of a "cover your a**" policy. Despite that many well designed receivers will easily handle the very benign 4ohm M80 load.

"Chickens don't clap."