So basically I've got everything cranked to the max and it's mega ****ing loud in the recording but when I export it from GarageBand and run it through iTunes, it's mega ****ing quiet. What am I doing wrong?
Probably because it's clipping in Garageband and you have it set to export at full loudness so GB kicks in and quiets it down. Take the full loudness setting off and you'll notice when you export it that the song will sound terribly distorted. You'll have to lower the volume in the mix.
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Is it just as quiet when you play it with another program (like an Apple-ish answer to Windows Media Player)?

iTunes has, by default, a setting that actually moderates the level of your audio. If you go to iTunes and right click on any track and select "get info" or whatever it is, there are a few tabs that come up. Under the "options" tab is an audio level option and you can increase it there.

Could I get some more talent in the monitors, please?

I know it sounds crazy, but try to learn to inhale your voice. www.thebelcantotechnique.com

Chris is the king of relating music things to other objects in real life.
I always had this problem with garageband and then i swapped to logic. Have you put a high pass filter on the instruments?
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