not required, as the bmw amp is set up to accept a high level input... you jsut on't get to turn the knob to MAX anymore.
Looking at your list, if you want to add a stereo to the 93 525i with stock speakers, the best thing you can do is actually dump the weak little amp section and let the headunit play them thru the factory xover.
20w x4 peak vs 40-50w peak. It makes a HUGE difference in clarity, above and beyond the gains from simply having a better head.