Thank you for the suggestions guys. I'm not sure if I understand correctly, but wouldn't a mic pre-amp only be able to split the signal that I'm already getting? I was thinking something that had two inputs to a single set of headphones like a Y connector would be perfect that way the inputs from both the mixer (what is being recorded) and from the computer (what has already been recorded and is now the backing track for the new track being recorded) can be heard together at the same time through a single pair of headphones. Does this make sense? Please let me know if there are any other suggestions.
As far as the Line 6 / Garageband combo, it's a useful setup because I can just plug in and record no problem without any of this stuff happening. There are limitations to both of these components that would have to be upgraded most likely, if I were more into recording. But I mostly use it for quiet practice, jamming with practice jam tracks, and some personal one track at a time layering, etc.
Have a great day! God Bless you!
Anthony