OK - so after spending a good hour playing (and actually getting time to fetch some of my CD's out of my old car to try out) I've ended up with:
Front/Back: F +4
L/R Balance - 0
This is with the dual-cone+tweeter-upgrade pack, so that might explain the differences. The really important thing is the front/back balance, though.
I started by rolling the fader completely to the front to get a good sound out of just those speakers, and +1 bass, 0 treble gives me that.
Then I brought the fader back to roll in a good solid dose of sub-bass, without letting it get out-of-control. The final sound is still not up to the old Bose I had in terms of pure sound quality (the mids and top ends are rather fragile and a bit strained at volume) but overally it's actually reaonable now.
This clearly shows that the gains for the rears are way too high, as you say, basegreen, and I agree the crossovers are set high too - probably to make things slightly clearer for passengers in the back, but who cares about them?
Anyway - I'm happier now, but I do agree that being able to adjust the gain and x-over properly seems essential - or at least have them set up by someone who's not deaf to begin with!
One last thing - a lot of the "wooliness" I was getting before seems to be down to the FM reciever, CD's sound far better. Obviously FM is something you should never rely on as a reference sound source, but I hadn't got the chance to try anything else as I've been a bit busy the last few days changing nappies, etc!
Oh, and while I tested with a reaonably wide selection of CD's, my collection is rather biased towards the kind of stuff you'd hear on MTV2, so if your tastes differ your results may too!
Next, when I get some free time - some tests with MP3/WMA CDs to see how they hold up.