You can't use amps, unless you have a preamp or a receiver acting as a preamp. My guess is a receiver alone would be fine, is this a new Sherwood? If so, it would be under warranty if it is new.
Of I remember, your room is not huge, so any decent receiver should be fine. Most people go with extra amps to drive speakers like the m80's, you will never need gobs of power for the rear channels in my opinion.
It is possible that your wiring is shorted. If it is running "under" stuff, how can you be sure that is not the problem.
Run some extra wiring above the flooring to the speakers and see if you have the same problem...
