d5000.5 sub output too low

[xannonite]

I have the following configuration, all brand new except for the head unit:

Pioneer DEH-400UB head unit
Polk Audio d5000.5 amp
Infinity 6030CS 90W 2ohm front speakers
Rockford Fosgate Punch 8" 200W 4ohm subwoofer

The front speakers work perfectly, but I am getting hardly any output on the subwoofer channel. Even if I set the sub to +6 on the head unit, turn the level all the way up on the amp, and turn the remote sub control all the way up, there's only a slight vibration on the subwoofer when I touch it. Switching the amp from SUB to INT increased the output a little, but it's still not enough to even make the sub audible. The SUBSONIC and LPF controls make no discernible difference.

To make sure there was nothing wrong with the sub itself, I replaced the d5000.5 with my old Planet Audio BB4.150 amp and connected the subwoofer. It worked perfectly at ~1/4 gain. Any ideas as to what the problem might be?

[xannonite]

Well, I called Polk Audio customer service, and they suggested connecting a second subwoofer in parallel to cut the impedance down to 2 ohms. I did so, and both subwoofers are working. They still aren't as loud as I expected them to be given they are 200W each, but they are good enough for now. However, I'm curious as to why the single subwoofer didn't work at 4 ohms even though it matched the RMS rating of the amp for that impedance.

[Closer]

Well, I called Polk Audio customer service, and they suggested connecting a second subwoofer in parallel to cut the impedance down to 2 ohms. I did so, and both subwoofers are working. They still aren't as loud as I expected them to be given they are 200W each, but they are good enough for now. However, I'm curious as to why the single subwoofer didn't work at 4 ohms even though it matched the RMS rating of the amp for that impedance.

I found out this amp likes a lot of deck volume or preamp voltage. I basically had to turn my deck 80% volume right before it seems to distort then I adjusted the the sub gain with the door speakers all the way down, then I just adjusted the gain on the doors to match. If you have 2 volt preouts that could be a issue.