Yes CRD. Nearly all amps and signal processors can take high level inputs (the actual speaker outputs from the Discovery Head Unit become the inputs for the amp / signal processor.
I have the speaker outputs from the headunit going into a signal processor under the passenger seat, the outputs from the signal processor go into a four channel amp and a mono amp. The four channel amp feeds the 8" rainbows in the front doors (and the tweeters via a crossover) and the rear speakers in the roof. The mono amp feeds the 10" Sub in an enclosure under the drivers seat.
I spend a lot of time driving my Kombi, it's the place where I listen to the most music. It's now the place where I've go the best sound!
I also sound deadened most of the van on advice from the many experts on the forums.