karma1959
Are you using 'entire mix' under export options? If so, that sums the track / bus outputs together I believe - which would change levels, given some tracks route to busses and others don't.
If all tracks and busses route to a master bus, select 'audio output' (or something like that - I can't remember the exact semantics and I'm not at my DAW right now) under export options. That should give you exactly what you're hearing from the master bus output.
Hope that helps.
Russ
This is the best answer so far - the entire problem sounds to me like a combination of improper routing & Export Options.
You MUST make sure every track is routed either to your Master Buss (Created by you) or indirectly via a sub-buss.
Choosing Main Outs under Source Category, Stereo under Channel Format, 44100 Sample Rate, Bit Depth of 32, Set Dithering to NONE.
Check everything under "Mix Enables" - you might need to uncheck Fast Bounce if you have BFD as your drum source (Also make sure that in the BFD gui you select "Offline")
Once you've completed the export, Import the Stereo Wave back into the project on a new track. Route this track directly to your Soundcard outputs (to avoid doubling up any master buss processing)
Now you can easily A/B between the mix & the export to prove they are identical.