Rockford fosgate prime r500-1

[Jeffl]

So I have a prime r500-1 amp my birth sheet says it put out 525 rms at 2 ohms, I'm trying to tune it to my head unit a jvc r-kd820bt which has 2.5v per amps, on the amp it's goes from 1(all the way left to 11(all the way right) I know ur suppose to tune it to your h/u pre amp volts, but it doesn't have a 2.5 v setting on the amp it's self, I've tried using a ddm meter to get 500 rms but that puts me at 3/11 on my amp gain and it barely plays. But if I turn it up to about 7/11 it hits a bit harder but it's way over what the ddm says it should be. Is my amp different since its a mono and a rf amp?