I've recently developed some FF5 issues.

The sub will continue with a low rumbling hum, with nothing being played from the source. There are also some occasional booms mixed in.

Pulling the power on the sub will reset it and stop the cycle , until it starts up again when receiving source input.

Pulling the Subwoofer input cable does not stop the sound.

Probably has to go in?

Hi TanLeng, make sure there are no interference causing devices nearby, like a wifi router or wireless phone. Can try moving the sub to a different room to verify the problem continues. If so, yes, contact your dealer with a description of the problem and the unit's serial number and they can get a replacement amp for you and get you fixed right up.