Sounds like an Auto-On issue. Try turning down the sub’s level knob, and then increasing the receiver’s sub output level.
A couple of things could potentially be causing this. If your TV’s outputs are variable, and the sound from the TV is noticeably louder than the other inputs, that would make it easier for the TV to trigger the sub’s auto-on. In addition, if you have the TV’s bass control dialed up, that would additionally contribute, as well.
Regards,
Wayne A. Pflughaupt