I have been pondering this too...but have been occupied with more pressing automotive issues.

Components completely aside - how are you planning to mount the components is the real issue. None of the speakers have true baffles or enclosures which means no bass, no matter how good the speakers are.

The most promising thing to do would be to mount a sub where the first air kit is, OR, put speakers in the rear door panels.

I'd love to come up with plans to make a nice W123 stereo that everyone can use...its not a trivial task at all. It requires some fabricating.

dd

PS - I talk abut all this with the end goal of a hi fi system - not a hugely loud raging sub driver thing.

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'85 300D, 'Lance',250k, ... winter beater (100k on franken-Frybrid 3 Valve Kit)
'82 300D, 'Tex', 228k body / 170k engine ... summer car
'83 300TD Cali Wagon 210k, wife's car