I completely forgot that I never responded to this thread.

Today I tried to get my subwoofer working and have only had a tiny bit more success.

I recently purchased this amp to power the sub, in case my current one was the problem.

Everything is wired correctly. A standard cable with RCA connectors goes from the subwoofer preout on the receiver (a Denon AVR-S510BT) to the left channel input on the amp. The amp is set to bridge mode with power set to "on" for testing purposes. The speaker is wired to the left positive and right positive as per the manufacturer's bridging instructions.

The result: a buzz. I'm getting a significant buzz from the sub. If I turn the amp's volume up to max, it's extremely noticeable. All the way down to around 10% it's pretty clearly audible. The buzz volume changes with the amp's volume, not the receiver.

I tried another source on the amp and didn't get this buzz. It's only from the receiver, whether the receiver is on or off. When the receiver is on, I can tell there's audio underneath the buzzing, and when I played a youtube clip with substantial bass, I definitely heard it. So much so that I'm even more frustrated that this buzz is there because I'm excited about how good the bass sounded.

I spent about 90 minutes on the phone with Denon and then Monoprice. The best we could come up with was a ground loop. They suggested I try plugging the amp into a different circuit than the receiver. I tried that tonight and it didn't have an effect. Should I even bother ordering a ground loop isolator? What do you guys think? Is there anything else I should look at?

Surprisingly, the Monoprice support was FAAAR better than Denon. That guy was trying everything, including digging into the Denon manual to look for ideas. The first person I spoke to at Denon listened to my problem, then immediately said "well that's a non-standard configuration, so I can't help." And I'm not really paraphrasing there. I asked to talk to a supervisor who didn't help much more but at least came up with the ground loop possibility.

I'm pretty annoyed, because this doesn't seem all that crazy to me. How much different is an in-wall sub connected to an amplifier than your average active subwoofer? Those just have all the components in a box, don't they?