SC_LSV, yes you can control the tower and cabin speakers (and sub) separately with the WS-420. You'll hook one set of RCA cables from the headunit to the EQ (input) and 3 sets from the EQ to your amps (output). 2 of those sets outputs will go to your M6600 and 1 set to the M1700.

A less expensive option is the Clarion EQS746. You lose the mic though. Either way I think an EQ is a better option than new headunit for tower control since you can simply turn a knob from the drivers seat.