Need help with diagnosing subwoofer issues - Monitor Audio R370HD

[Mithrandir]

If the volume goes intermittently full level and uncontrollable by the volume control, I'd actually suspect the volume control itself.  Another possibility is the solder joint for the CCW connection of the pot if it's wired in the common voltage divider way.  If the volume control is wired in the feedback path of an opamp instead, then an open circuit control could also give that symptom.

I don't think I've ever seen a subwoofer with an AGC as why would you ever want that.  Similarly I don't think it's a mute line issue as that would only cause a reduction in volume.  This is all assuming your statement that the volume goes full and can't be controlled is accurate.

I'm not 100% sure it's full volume but it goes very loud. Volume control is maybe on 30%.
Whenever it goes loud volume control doesn't work at all. I can turn it all up or down and it doesn't change volume.
When that happens I have to turn subwoofer off/on and then it works correctly again.

I'm a newbie so forgive me for asking, but what's an CCW connection?

[David_AVD]

CCW means "Counter Clock Wise". 

It is odd that the unit resumes normal operation when the power is cycled.  I missed that in your original post.

It would be a lot easier to guess what may be wrong if we had a schematic.