d5000.5 sub output too low
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?