you can't drive and amp with a line level (and it's so noisy)
Yes, you can... FF400 output -> marshall VS100 -> mic's -> FF400 input
http://www.myspace.com/winterlunium
If he doesn't get anything near an ok sound just going from line out directly into the amp - The audio interface is not giving the right level signal.
EDIT: ... Maybe this is different with a 5150 .. in that case I'll shutup ... My VS100 did not mind the line-level at all, though...