Ok so the DM-810 takes the signal from the head unit and bypasses the stock Sony amp and stock wiring. That would really be the cleanest sounding way to go?
The DM-810 is simply and 8-channel-in/10-channel-out processor. Once you grab the signal you're going to route it from the processor to amplification and speakers of your choice. I'd take it to your car audio professional and have them do a work-up on it.