No tweeter for your E34?? I thought the tweeters are hidden in the dashboard. Changing the speakers is a very straight forward job I think most accessory shop would be able to carry out.
I have my front speakers changed from the stock Nokia 5.25" with 1" tweeter to Focal Polyglass165V2 6.5" component. Slight mod job to the door to accomodate the bigger mid/bass driver but I'm glad I have done it.
The rear speakers have been replaced with a cheaper brand 6.5" component but taken out as I feel that the new front speakers are doing the job well.
I have also changed the Nakamichi tape and tuner head unit to Alpine and threw away the tiny sized Nakamichi power amp and replaced with Lanzar.