Not entirely sure how you are monitoring the output, do you have an external mixer, headphones off the Soundcard ?
If you export just "tracks" as source category , I think you will not be processing any plugins you have added in the project, just the raw wave files.
Perhaps you should solo the tracks (or mute others) you want to export, then select your master bus as a source category, making sure this master bus is directed to the soundcard out ? Also checkbox any plugins etc being used. Then they should match in volume etc.
I'm no expert at mastering, but from what I've read here, you should send out files with minimal compression, and effects, with plenty of headroom for the masters of mastering to add their thing. Usually "mastering" will be a fine tuning of existing complete mixes, to make them sound sonically similar.