Never had a Z3 with the Harman Kardon stereo but I have seen it fitted in the front facing storage compartment between the seats.
My query is do you also lose the top storage bin or is it just the smaller front facing one?
View attachment 242777
The speaker imaged is the subwoofer, mine has one, but it is not HK. My driver was rotted out so I replaced it with 2 drivers. I think the HK version is ported, whereas the Phillips I believe, has a drone cone forward and rear facing woofer. An ideal configuration to replace the drone cone with a signal inverted second driver...But less than ideal in that the rear facing driver has no protection against water ingress, if you get caught in a shower, top down.. Either way, both versions eliminate any storage at all.