Depends on your priorities. If you like bassy sound, and don't mind sub sound overtaking other drivers, get the rear sub.
However if you prefer balanced sound, then you'd require all of it updated.
I'd say start with sub in the boot and take it from there.
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F36 Xdrive for her - HUD, park assist, heated steering wheel, rear camera, Apple car play
F36 RWD for him - HK, M sports pack, GTS tail light, Apple car play
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