Heres my advice / current set-up;

Becker 780 in the dash. It uses the factory connectors for all W123's, but it was a late 80's model with better internals.

Pioneer speakers work great in the W123, 4" in the front under the factory grills, and 5.25" in the rear.

The sound is more then adequate. There is some bass, but remember, your biggest speaker is 5.25", it's simply a small speaker.

I am very happy with that set-up in mine.

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Adam Lumsden
(83) 300D
Vice-President of the MBCA International Stars Section