Paul, this is a similar idea to one that I have been messing around with for a little while. Like a few major car audio companies you should add a microphone input so it will do real time analysis and phase compensation between speakers. Are you going to have the DSP do real time filter coeficient generation? Also, have you determined which DSP you will use? My personal favorite is the Analog Devices SHARC DSP's. I believe they are currently the fastest for FFT's. I think they have a low cost 21060L series with 1Mbit of memory that is only about $10 if I remember correctly. The upper models will run a few hundred dollars but you probably don't need that much memory/features.

Alex Lear