Could be the amp, but before you get down and dirty on the amp try a new cable. Also, does the sub input connector on the sub and receiver seem to be 100% solid? No wiggling or back and forth and the cable is snug both sides?
butter and jelly please.
Home theater gladly not designed by ereed or bgtighe23. 65" JS9500, Marantz 7702MKII, Parasound Halo A31, ClassDaudio SDS470 4ch, RBH R56, 4xRBH R55, 2x RBH R5, Hsu VTF 2 MK 3 and VTF 3 for subwoofage, OnStage MS7701B mic stand with boom for improved Audussey XT32 calibrations. Listening space happily treated with 1" OC 703 rigid fiberglass. OPPO 203 or 205 soon.