Having difficulty with quality when I export to iTunes

Question

My faithful GB geniuses:
I've been using GB for quite some time now and have not had much trouble when it comes to sound quality. I know that when you export to iTunes, it mixes down your song to a file with the resolution of 44.1 kHz at 16 bits (I may even be confused here -- no matter, these numbers mean little to me since I do not fully understand sample rates, etc).
Anyway, I have this song that has eleven MIDI/Software Instrument tracks and six Real Instrument tracks (5 guitars, one bass). It sounds great when I playback the GB file, during editing. Then, when I Export it to iTunes, there is one guitar (a lead guitar with Amp Simulator, Distortion, and Echo effects added) that comes out sounding like utter crap. When it comes in, the entire file sounds crackly and awful. The guitar drops in volume, and also quality.
Up until the distorted guitar track comes in, the song sounds fine. So it's not the other 5 guitars that's doing it. Does my problem have to do with the fact that the lead guitar has too many effects on it for GB to handle? But no, this can't be right because when I play the GB file, it sounds studio-quality. When I export to iTunes, it sounds like somebody is slowly murdering a cat.
Also, I went into iTunes prefs and changed the "Importing" settings to "Highest Quality", and it did not make it any better. If this problem can be remedied through some simple changes in iTunes Importing prefs, can somebody briefly explain to me how to do so?
Thank god I have you guys. Help me out! On a side note, what's with all these people complaining about how hard GB is to use? With the exception of my case, in which I think I'm trying to push the bounds of what GB can accomplish, GB is ridiculously easy to navigate.
On another side note, check out my songs that HAVE come out successfully, quality-wise!: www.soundclick.com/dijitel.
Thanks in advance for your help, guys.
dijitel.

Answer 1

Thanks for the input, AppleGuy.
It seems my initial problem was that I had the EQ on some setting on iTunes by accident, and that was affecting how some GB songs sounded once they were exported. Then I tried locking the tracks (all of them actually), and doing it again, and that didn't seem to make much of a difference since I had already solved the problem.
But that's an interesting point -- what sort of effect does locking the tracks have on how they sound once they're exported to iTunes? Should I lock all of the tracks now before I export?
Any other basic tips on getting the absolute best quality possible out of a song that has a lot of real instrument tracks is welcomed. Thanks again for your help.
dij.