pioneer amp clipping?

I have a pioneer GM-8400M ( 300Wx1@4ohm, 600Wx1@2ohm) amp, and i just started testing it out to make sure it works, but it doesn't seem to be working right.  I have already tried calling pioneer tech support, but its been busy everytime i call.  The problem is that the sub will pop when the amp plays on what seems to me to be very reasonable volume and higher.  I need to have the gains set to around 5-4v (maybe a quarter of the way up) (radio puts out 4v preout) and the bass knob no more than half way for it to play correctly, but at that point, it is very low.  At first i thought it was that the sub couldn't handle the power (JL W0 @4ohm) so then i hooked it up to a Kicker L5 @ 8ohm, and it did the same thing.  We then tried it at 2ohm, but no success.  Do you think there is something wrong with the amplifier, or is the amp actually supposed to be putting out good power, and the subs can't handle it? 

Also note:  We have been testing this amp on a test bench, using a 10a power inverter and a cap wired into it, however any other amp we test seems to put out more power and work fine on it.

i'm gonna put it in a car and see what happens with a little more current going into the amp.