Also how are you running the voltage wires?I just bought a cheap
ebay gauge (wish I had spent more and gotten the 3 wire Stinger) but I was going to hook it up behind the dash like I think you have done but that voltage can differ from the voltage the amp is getting.
So for me I am going to ground the voltmeter behind the dash and run a wire to the rear of the car.
Back there I am running a relay to turn on both of my amps (triggered from the remote wire, feed power from the rear Dist block) and I am going to connect the power wire to the voltmeter to the relay output (the same wire that turns on both of my amps)
That should give a better reading of what voltage the amp is getting.
edit** that sounds harder than it really is.