Very Informative Craig! One thing I wanted to add is that the DIgitech GSP line has a setting in the unit that shapes the signal based on the type of amp, such as SS stack, Tube Combo, Tube Stack, etc.. that can make all the difference in the world- as these units do not come out of the box preset to work with tube amps. The usually come with the cabinet simulators turned on, and the shaping set to direct (PA), and when plugged through a guitar amp, as you know, sounds like garbage!
I didn't realize the part about the input levels, but have had that digital distortion (I called it digital