cab,

You could potentially damage a dual voice coil woofer by wiring only one of the coils. You would be inordinately working one coil with half the overall thermal capacity.

The Polk MOMO sub is very anemic in a sealed enclosure but will sound absolutely great in a bass-reflex enclosure. I've never experienced another woofer where the performance was so polarized. In a bass-reflex enclosure you would get roughly the same output with 200 watts as you would have in a sealed box with 400 watts. So if you're sealed now, then give this some consideration.

You're correct that a dual 4-ohm in parallel is the way to go in order to draw the full output from your amplifier. Also, I totally agree with all of Brian R's comments. Very good points.

David
Earmark Marine