Need some info, 97 740i

So I bought a 97 740i last week, and I would like to upgrade the sound system. I would like to upgrade the deck and add a single sub if possible, and down the line maybe the speakers and a speaker amp.
Ive done a bit of reading and have seen some conflicting reports on what works and what doesn't. Does anyone have a good diagram on how the sound system is hooked up and how everything integrates with I-BUS?
My car has the 6 disc changer, and DSP button. The main thing I would like to know is how the deck sends signal to the factory amp and speakers, from what I have read, the main volume control interfaces through I-BUS straight to the amp rather than altering rca voltages, is this correct?
The previous owner supplied me with a harness that takes the speaker outs from the new deck and splices them right into RCA connectors, but I don't see how that would work with the volume control already in the car. Would the stock volume control and the new deck volume control both be controlling volume in this case unless the pre outs from the deck were set to a fixed output?
Having three units; the tape deck part where the body harness comes in, the dsp/EQ module, and the unit with volume control, and screen for vehicle info stuff has me confused. Any help would be appreciated, thanks.