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...... :)

Tweets not for bass
 

I don't do the tweeters for bass...just cos they were there and I wanted to try them...as it was it kinda brought the rebel sound up behind like surround sound so it was a nice little mod....

Compared with my previous car, the sound in my D2 FL is leaps and bounds ahead of what I had before. The Bose and Symphony headunit seem to go well together, and sound great if you play a good quality CD through the system.

I thought the tweeters were missing or broken in my rear doors until reading the PDF earlier and now realise they are meant to be missing.

Bose sound in the A8's
 

In general the sound in the A8's is what I would call "aims to impress" but in truth it's rather poor. In many things in life more is not always better, and one thing for sure there is far too much bass in these systems, it even plays bass notes that are not there, the speakers are running in self resonance. At the other end of the spectrum the treble is too strong so it gives a very false sound , just listen to the news - did you ever hear someone's voice sound like that?
There are many reasons why I have owned A8's but the sound system is not one of them. Audi have partnered with the high brand profile of Bose, it does not mean it is the best in car audio or home audio for that matter.

Another point to make is correctly balanced sound- Bass Vs treble they do interact in an opposite way you would think. This is because bass sounds are rich with harmonics, these harmonics are in the high frequency region. if this were not true the top string on a bass guitar would just be a 67Hz buzz with no tone.
My final point is a question you should think about - consider a male singer and a typical female singer which voice has the most treble content?...........