I think the mastering stage is only really the final polish, and wont fix any major problems.
If you are having problems with the bottom end in your tracks then the problem should be fixed before the mastering stage.
Good subtractive eq is the thing.
EQ is so important.

I may be able to help you a little with improving your mixes, if you post some tracks in the Techno Production Filez section of this forum, I can have a listen and try to point out what needs to be done to get your mix to that final polish.