This could be difficult to do for under $2000. Do you just need the M-80's or do you need to buy all the other speakers and a receiver to get 7.1?
M80's, even from the outlet are $990 US. As far as receivers that will work with 4 ohm speakers I own a
HK AVR-7200 that will power the M 80's and I've seen in other posts that the yamaha rxv-2400 can handle 4 ohms as well. The harmon kardon is very reasonably priced at the moment ($750-$800 or so) as they will be releasing their new top of the line AVR-7300 soon. Too many questions not enough info right now.
