GBMINI stereo – recovering from Tweeter

I dismantled the rear interior of GBMINI (again!) today – removed seats and both rear panels – and relocated the Kenwood Keg from the right to the left; I was then able to re-install the Harmon Kardon amplifier in its original position. I also stripped out all the surplus wiring that Tweeter had left behind – I discovered that they had replaced my $100 RCA 6 way cable run with a less expensive 4 way cable, so I guess I have some more complaint letters to send them!

For anyone trying to follow GBMINIs stereo, here is what I have today:
The Kenwood KDCX-759 head unit is installed in the car:

The Kenwood Keg (shown above) gives me 20GB of MP3 music storage (probably enough for every CD I own); the HU display scrolls the “track title” when the Keg is playing (which might be the name of the track and artist, or maybe the track, CD title and artist, or whatever I configure when I load the Keg using the PhatNoise software. Of course, the HU can also play radio or CD.
The factory Harmon Kardon amplifier is installed in the car, driving factory HK speakers – in each door there is a bass, mid and tweeter (the mid/tweeter pair are driven from a separate amplifier signal to the bass); in each rear panel is a dual voice coil 6×9 speaker which mainly does bass but in some HK modes extends into the midrange.
The HK amplifier receives a stereo speaker level feed from the front left/right output of the HU – the rear HU speaker outputs are not used (the RCA outputs are also unused). So the fade control on the HU serves no purpose!
The HK amplifier expects to be driven from the factory HU; it receives volume, balance, fade, bass, treble and “mode” signals from it and it expects a speaker level which does not vary with the volume control. To make this work, I still have the factory HU installed in the car (currently laying in the trunk); without this, the HK amplifier will not turn on. The “proper” way to drive the HK amplifier would be to adjust the Kenwood HU to a volume level which puts out a speaker voltage matching that of the factory HU, then use the factory HU volume control to determine the volume in the car!
In practice, I can currently use EITHER volume control. However if I turn the Kenwood HU up too loud, I might overload the HK amplifier inputs which will distort the sound (but I have not noticed this yet). Alternatively if I turn down the factory HU volume too far, I will get no sound at any Kenwood setting. Another limit is that if the factory HU volume is at max, there does not seem to be any “speed compensation”.
I am experimenting with different volume ranges for the two HUs; I hope to find a fixed setting for the factory HU which I might then be able to leave alone, and use just the Kenwood HU to vary volume over an acceptable range. My plan is to make an electronic circuit which mimics the communication between factory HU and HK amplifier, so the factory HU can be removed.
The HackTheIBus Yahoo forum has interesting details which might help me decode the communication – I need to find more time to investigate!
I am also experimenting with the HK amplifier “modes” (MINI H/K, SPATIAL, ELECTRONIC, etc) which this web site discusses in detail. Todays favorite mode is SPATIAL since it gets closer to surrounding me with audio, which is how I prefer to listen to music.
Am I crazy? Probably! Many people would be happy with the Harmon Kardon system as delivered by MINI. I wanted more: I wanted the Keg, with a HU that could display the track information. I hoped for better sound – hard to claim that replacing only the HU has made any improvement since the HK sound system is very dependant on the software algorithms in the amplifier.
Some other MINI owners have successfully replaced their sound system; at least one person replaced the Harmon Kardon amplifier and claims to have a better result, but my experiences with Tweeter have scared me off that for now.