I have a very old EP350 sub and it suddenly started emitting a loud rumble with no source turned on. Tried adjusting all settings, switched power outlets (including trying it on a UPS), attached and removed the RCA cable. If I lower the volume I can still hear the rumble, but at 70% (my calibrated setting with my AV Receiver), it's unusable and I have to turn it off. Is this the end of life of the sub amp?