OP...was this ever resolved?
Quick summation...
Equipment
1)Dual Voice Coil Subwoofer
2)Two channel Pioneer amp
Fix
Bridge the amp and run the coils in series.
Reason
The Pioneer amp is stable at 4ohm when bridged. The Voice coils MUST be run in tandem. Meaning they must receive the same signal or else you'll jack up your sub [refer here: http:]//electronics.howstuffworks.com/speaker5.htm
Therefore, you must run the coils off the same channel. To get the most out of your Pioneer amp you would need to bridge the channels. BUT!!! You're Pioneer isn't stable below 4ohms when bridged. That's why you must run the voice coil in series. [refer here: Subwoofer Wiring Diagrams, One 2 ohm Dual Voice Coil (DVC) Speaker]
