Ok discussions with a variety of folks finally yielded some useful information. It turns out the E89 676 HIFI sound system is pretty on par with the F30 676 HIFI. I've purchased a plug and play amp solution from
bimmertech, and spoke with Nelson there on the details. An unexpected advantage of this amplifier is the ability to use
tuning software to configure how the amp is configured. There are folks trading configuration files for various cars with various speaker setups. Of course the E89 Z4 has this forum (near dead) with not much of this kind of activity. But Nelson told me they've done a number of Z4's so they'll provide me something to start with, and I'll work it from there.
Best as I can tell, talking with friends, the footwell speaker is a 16.5cm (6.5-inch) woofer. I'm going to pull a panel and measure it. If that's correct, I plan to install a pair of
Hertz 165.1 speakers. Combined with the remaining speakers being Bavsound Stage 1, I believe the complete setup should be quite adequate. I'll report as I make progress.
Filippo