New Logic User - Can't stop getting feedback on my guitar on a mac pro!

Hello all, a friend bought me a copy of Logic 9 as a gift to pay me back for some free work I did for him (I'm an editor). I was so excited to try out the amp and pedal simulator in real time (I'm a guitar player) and maybe be able to sell some of my old gear if this thing sounds good enough... - so I figured out how to do it in Main Stage but all I get is feedback! I'm running my guitar into a Behringer 8 track mixer and then out to a Griffin imic. I can get a signal from my guitar and it's clean. I setup the audio device in my preferences panel of Logic and Main Stage. I've also tested my connection in Final Cut with the VO tool and recorded perfectly clean guitar so it's not the imic malfunctioning. It seems that in Main Stage on my Channel Strips, when I unmute the "Output 1-2" strip, that's when I get the feedback. Right now I have my output set to "built-in line output" which is running back into my mixer and then to my Maudio monitors. One more quick note, I noticed that if I play music in itunes, the levels show up in Main Stage under my "guitar 1" channel strip which didn't seem right to me since that should only be my guitar levels coming from the imic.

Please help! So frustrated! Here's my system specs:

Mac Pro Quad 3.0ghz
12 gigs of ram
10.5.7
Logic Pro 9
imic

Thanks guys...

FCP / AVID EDITOR
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