Optimally, your Denon sub level should be approx. at 0db, and the volume knob at the sub itself, while looking at the amp plate from behind the subwoofer, should be at approx. 7 to 8 o'clock. Anything beyond that would be overpowering.

Epic 80 / SVS PB13 Ultra
Denon 3805 / M2200 Outlaw Monos /
Sammy 55" LED