I will keep stock radio/head unit in the E39 530i because the car computer is too tightly integrated and I want to keep those functions. I am using a GROM Audio integration unit to give me USB and smart phone inputs to the stock radio right now. If I add an analog preamp, I will not keep the GROM unit installed. I will probably install a ZAPCO 4 channel DSP amp in the trunk for front and rear speakers and am undecided on subwoofer amp. I will put a compact sub in the trunk firing through the rear seat armrest cutout using a pair of Peerless SLS 6.5" drivers. They will be more than enough for my listening habits. I have a set of new in box Dynaudio components that I will install the front doors. I have a set of custom A Pillars for a 3" driver and a tweeter from a previous install but not planning on using them in this car unless I don't like the Dynaudios. As far as the analog preamp, I am going to look into removing the OEM CD player from the stock head unit housing and putting the preamp there. That would let me retain the on board computer and keep the dash looking as stock as possible. There is a fold down wood trim flap that would hide the preamp if I am successful.
I probably going to install a Pioneer DEH-80PRS in the 97 328iC and am undecided on which amp(s) and speakers to use in this car. It is a special edition model with stitched Napa Leather interior and I do not want to cut door panels or kick panels for a custom speaker install.
The 95 525iT is still stock and I am in the midst of swapping an M3 engine and tranny into this car. The audio install will have to wait until I get the mechanical work done.