I believe all W140s had the BAose system. I have a 1992, my rear ( 4 subs across teh rear deck were cracking, squealing etc. Bad amp just the left rear bass channel. I do have poping and scratching intermitently form the right rear door speaker.

I Put ina new head unit ( Sony ) with a 10 disk CD changer ( car didn't have a CD cahnger. I used teh Stock Bose amp for all 7 forward speakers - front and rear doors and center channel.
I install an aftermarket 2 cahnnel amp for just eh subs.

Sounds awesome. I did it my self.

You can use the factory HU and ad an aftermarket amp.
You need adjustable line level adapter/convertrers.

Scott for La Jolla got on teh straight and narrow on this one!!

Steve
from Toronto,Canada! EH