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

@beep - when you insert mono tracks, or drag in mono clips, stereo interleave is turned off - because the audio is mono. what i hate is if i take a mono clip in a project and drag it to my media browser it converts it to stereo.  ugh. HATE HATE HATE IT!

Main Output as export option?

Cool. Thanks Russ. Just following the X1 Power export section on this which is surprisingly sparse compared to the rest of the book. I thought it might be something like that. I'll mess around with it some more. Cheers.

@UKJK... I'm thinking I might try and export using Buses (I guess that's called stems) and Main Outputs. I'd kind of like to see what happens with both and maybe have the individual buses will let me get the mix closer to where I want.

  We haven't met yet but I'm kind of a beginner so this stuff is all a little new to me. 

Thanks again Russ for giving me some ideas to check out. Cheers.

Karma, that's certainly a different sounding .wav output to the Entire mix option, I just tested it. Will need to try again in the morning, I'm too tired now, but that might well have been my issue all along. Cheers

@UKJEDI & beep - ok, one thing you need to watch out for when dealing with audio is this... i'll give you an example too... 

on the audio track, open the inspector and click the track tab.  you'll see a header for audio snap with two options - online render and offline render. 

here's where the example comes in...
leave both settings on default.
with a small audio clip, control rightclick the right edge of the clip (it will turn yellow) and stretch the audio to 400%.  play it back. you'll notice a glitching stutter. 
now, right click the clip and select bounce to clips. 
play it back again. notice the difference???

ok, so undo the bounce you just did - play back and you'll hear the stutter again because it's a stretched clip. now, on offline render in the audio snap options, click and select "same as online". right click the clip again and select bounce to clips.  take a listen...
this time the stuttering is preserved with the clips.  
i've noticed this track setting can make a difference (small but noticeable) when dealing with looped content and linked or imported audio with tempo or pitch info.  

try changing all your audio tracks to "same as online" to see if that makes a diff.

Well the Main Outputs option is creating two separate waves it seems. Not what I was expecting. Guess I'll have to import both to the mastering session. I just don't understand why this wasn't working the normal way. Man this crap is confusing. :-/

@BEEP - make sure channel format is stereo not split mono.

Welp... that did indeed work but in a weird way. The second file it created was my Mon 1-2 mix and it sounds the way it's supposed to. Not sure what the other export was but I'm guessing it might be the ADAT out or something. I don't really care right now because I have what I wanted but I'll check it out later out of curiosity. If all goes well I'll have this tune up on the songs forum later tonight. Cheers and thanks again, Russ.