I would definitely try disconnecting the sub altogether.
If I understand correctly you are connecting the two amps to a single powered sub (i.e., both amps are connected to the same sub, with the sub providing speaker-level inputs for two channels). If so, it is likely that there is a ground path within the sub that is connecting one of the two positive output terminals of each amp (the output terminal of the amp that I presume is connected to a negative input terminal of the sub) to the corresponding positive output terminal of the other amp. Which could very likely account for the problem. And that path would exist even when the sub is turned off.
Regards,
-- Al
If I understand correctly you are connecting the two amps to a single powered sub (i.e., both amps are connected to the same sub, with the sub providing speaker-level inputs for two channels). If so, it is likely that there is a ground path within the sub that is connecting one of the two positive output terminals of each amp (the output terminal of the amp that I presume is connected to a negative input terminal of the sub) to the corresponding positive output terminal of the other amp. Which could very likely account for the problem. And that path would exist even when the sub is turned off.
Regards,
-- Al