KB, welcome. It shouldn't be stated categorically that the 5660(or any other receiver)won't be adequate for a setup with the M80s. This depends on how loud of an average listening level is used and how wide the dynamic range of the material being played is. Speaker manufacturers(and receiver manufacturers)don't like to be blamed when a particular speaker/receiver combination keeps shutting down and they may be overly cautious in some of their suggestions. There have been a fair number of reports of successful use of M80s with moderately powered Yamaha and other receivers not specifically recommended for 4 ohm operation. If you really want to try the M80s, consider doing so if you're willing to send them back if they're not satisfactory for your use.
