BMW used a stupidly under-powered Harman Kardon subwoofer in the Z3.
You can buy a repair kit for the paper cone. I decided to replace the entire sound system, including the sub.

Subwoofer: Kicker 44CWCD84
https://www.crutchfield.com/p_206CWCD...

Head unit:
Sony MEX-N5200BT
https://www.amazon.com/Sony-MEX-N5200...