[Night Train]

The tweets added into the rear doors shouldn't affect the bass in any way, as they should have chokes fitted & the rear door speakers, like the front door ones are only mid's anyway.

I'm considering putting some Kef tweets in my front doors and moving the Bose ones to the rear doors, but very attenuated, and also fitting some more Kef tweets in the panel where the sunroof control is, on a custom 20w amp (again attenuated) to raise the soundstage (main) in the front. (as fitted in my P5B)
The rear stage is only 'fill in', other than the bass which is from the rear shelf in the D2 as standard. (again considering refitting my 3 x JL Audio 12W6's ported through a ski hatch possibly, but that might be overkill in the S8)

£1200 would buy 2/3 of one of my home stereo's speakers, rather than a handbag, I've never understood the 'fetish' for shoes & handbags, especially 'designer' labels......