I generally agree with G. Wood's comments. Mastering does affect the overall volume level but it also can shape the sound of each track. Shmo, I can't tell from your post whether you attended the mastering session, or whether you sent the tracks to someone to master and they sent them back to you. I recommend that if you get the opportunity to attend a mastering session, that you do it. And while you are there, ask all of the questions you have about the mastering process. I also recommend that you have the tracks mastered by someone other than the person who mixed (and/or recorded) the tracks.