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Unless it has always had this problem with these speakers, you almost certainly don't have an impedance matching problem with the speakers... and the other symptoms wouldn't be caused by that anyway.
Check your antenna connection. Lack of antenna could cause some of the problem if not all of it, and would certainly cause the FM to be hissing and nothing else.
Otherwise, your account clearly indicates a severe electronic problem in the Denon unit. A component in the amplifiers stage(s) has gone bad, and your Denon device will require repair.