Quote Originally Posted by gmannino
Without replacing the amp or the factory head unit, even with new speakers or upgraded speakers, you will not notice a difference in output. You might notice a slight increase in clarity but you will not increase the output. Upgrading to a larger speaker in the rear might even decrease output since more wattage is required to drive the larger driver.

In order to increase output you need to add more wattage to the system. This can be done in only two ways with the e34. Replace the factory amp or replace the factory head unit.

Replacing the factory head unit with a good aftermarket unit will have a very noticeable increase in output and clarity. The factory deck in the BMW is considered the weak point of the audio system and by replacing it, you will notice that extra output and clarity you were looking for. Some people will say that factory radio is not a weak point but they are simply mistaken.

I have installed thousands of audio systems in BMW's and replacing the head unit alone always made a good increase in sound quality and output. Replacing the amp and the speakers are to be done at the same time.
one caveat: replacing the head will not increase wattage... you just drive the 20x4 peak amp with a cleaner, more dynamic, and larger signal. it is still only 20x4 peak.

IF you are lucky enough to not own a MY94-95 you maybeable to 'bypass' the amp and use the more powerful headunit amp section. That does make a huge difference.