well...as usual, bucking common sence (just buying new gear) I went ahead and bid on the old Alpine sub. self enclosed ported 10" sub w/ built in 70 watt amp. $41.00 shipped.
I was thinking of my head unit going to an EQ with a sub controller output. feed the sub from there and also a 4 channel amp for the 4 doors.
maybe I can just make a switch for the head to EQ line... from there, switch between the head unit or minidisc ?
I don't know much about the details of stereo systems, but I think this would work....?