I need help adjusting level on amp more deatails in post!?

Ok so I have two Alpine type r 12" 500watts rms each, a hifonics Brutus bxi. 2008d with is 2000watts but is running at 1300watts rms at 2ohms, so yeah my subs are 4ohms but wired down to 2ohms and I have a jvc kd-hdr40 headunit. In the manual it says something like this: lineout level 2.5vand than subwoofer out level 2.5v so I put my amp at 2.5v but don't know if that's correct! Also I want these subs to get loud to their peek but still sound good I have put the lvl to 1v but they couldn't go louder