kenny,
The addition of the WS-420 will give you 3 zone volume control: tower, sub and entire in-boat, meaning main cabin and bow as a single zone. if you desire bow volume control, I would suggest running the 420 input from the head-unit's rear output, then run the bow amp inputs to the head-unit's front RCA outputs with a simple line level POT in between. locate that line level controller in the bow area. head-unit volume will typically be near 100% and the 420 will serve as master volume and line level for bow controls their level independent.
I would work to keep the woofer where it is and work the heater so an enclosure can be placed in there along with it.
Michael
Mikes Liquid Audio