sorry I didnt get back to you sooner...ok I don't think we are on the same page here.
What I'm suggesting is to have the Mbox connected to your PC as usual(via USB) launch your recording program and record a dry track...ie no distortion or anything. Then take the track and bounce it to a mp3 or wave format. Next, connect the output of the speaker from your pc directly.(headphone jack out) into the Line 6 input...where you plug in the guitar. Then hit play on your pc and listen to the track with the new sound thru your Mbox.... hit record on the program again. It totally works...I've used this method before.