Check out the Onkyo TX-SR 601 (you can get it as low as $300). I have one and love it. There's a very postive review of it driving the M80's here, so you know it can drive 6 and 4 ohm loads. Just make sure it's not in a small confined space so it can be allowed to stay cool:

http://www.audioholics.com/productreviews/avhardware/onkyo_TX-SR601_review_01.html

Particularly, check out pg. 6.

-JT

Fight on 'SC! Three-Pete Baby!