low output from sub

Hi

I am running a Rockford P312D4 Sub in a band pass box.  I am pushing it with a Hifonics Neptune with the VC's in parallel to produce 500Watts at 2 ohms.  My head unit is a Rockford Fosgate RAV-DVD1 which says it puts out 5Volts.

I have the gain up all the way on the amp, the bass boost is half way up, and on the head unit the "punch level" and bass adjustment are also higher than 0.  My output is still not near what I was expecting.  I used to run a couple of single VC old school paper cone subs at about 150 watts and they hit a lot harder than this. 

My bandpass box sounds great when I do get the bass to hit, but I suppose it may have been built wrong,  It still seems like a box that is slightly off in volume or port dimension would hit harder than this. 

Is there something I am over looking?  I am a dumbass as I am just getting into this.  Thanks.

I guarantee that amp is clipping with the gain at max and it's not surprising that you can't hear it in a bandpass.  Run like that for long and you certainly could fry your speaker.  The suggestion that your VCs might be out of phase is a good one.  Or maybe the speaker is simply blown already.