track confusion in fast track ultra interface

[deck]

So... I'm fairly new to all of this digital recording business, but I'm using PT 8 with a Fast Track Ultra interface on a 64-bit Vista. I'm just recording instrument tracks so far and have it decently figured out, but i have run into a problem that has me stumped. I am plugging a bass and a guitar into separate inputs on the interface through the external speaker on each ones amp, and the sound registers on separate tracks for each instrument, but I am also getting a bit of the guitar track recorded onto the bass track. The sound of the guitar on the bass track is really crunchy and far away sounding. I can not figure out how to get rid of it. I have tried switching inputs on the interface and still no dice. Any advice would be helpful.