so just use my dmm and put the + and - in the power and ground spots on my amp?

Ya and look how much the voltage drops from standing voltage to when the bass hits.

if i normally listen to it around 15 or so should i reset my gain with the volume around there?

If that's the highest you listen at, then yeah, set it there.

I would go a bit higher, that way if you show off no harm done by possibly clipping the signal... So shoot for volume of 21 and set the gain there or maybe 19... That way it will be good at your normal listening and safe for going louder???