Brundelfly - I think I'll stick the zero db option on dim solo in as a feature request and see what happens.
FastBikerBoy - what you suggests does work and that is in fact what I'm doing at the moment. It's just a faff to have to keep setting and unsetting the levels of the comped track and the master bus.
neirbod - if you do manage to throw more light on things I'd really appreciate it.
Grem - "what is speed comp'ing"; in x3, after you've got a few takes in take lanes, you can split it up into segments, select any one of the segments then hit shift + spacebar and playback will start with looping on that segment. While it is looping you can use the up and down arrow to audition different takes, once you've found the one you are happy with then 'enter' will promote it to the comped track. Forward and backward arrows will move you to adjacent segments to repeat the process. It seems to have a lot of potential, unfortunately it goes ahead and solos the comped track for you and that particular 'feature' is driving me mad!