aftermarket head unit

Hey everybody. So I'm getting a Pioneer DVD in-dash head unit (AVH-X2700BS). It has rca sub outs. For now I'm gonna keep the factory sub until I can build the box I want. I've looked into using RCA-to-speaker-wire adapters (or modifying an RCA cable myself) directly from the sub to the back of...

I'm looking at installing a Kenwood DPX-500BT head unit in my '08 Ridge RTL, no nav. A note on the Crutchfield site says the subwoofer will not work with this unit, and with many other aftermarket HUs. Anyone know why, and most important, if there's a work around? (I'd rather not have to spring...