Correct me if I'm wrong....
Can't you run 2 sets of RCA's from the 420 to the Syn4 powering in-boats and sub, one for the in-boats on the rear channel and the other set for the sub on the front channel?
Bridge the front channel for the sub. Stereo high pass with mono sub.....page 5. http://www.wetsounds.com/media/produ...ser_Manual.pdf
I'd like to see you get those 2 bow speakers off the headunit too, which in-boats do you have?