The gain knob is used to control how much stronger/weaker the incoming signal will be pre-amplified (gain) before the amplication that will drive the sub's woofers.

After re-checking your AVR settings (I think you have a Denon, so a Denon owner will need to jump in at this point for this part), try playing something with heavy bass and slowly (very slowy), turn the gain knob up.

See Mojo's signature