As it's a preamp it will probably be closer to the 5150 tone also has it's own clean channel I think, normally I think most distortion boxes always lack something these AMT's seem to have more oomph...
Actually it has three outputs a cab sim out for direct a preamp out for going into an amps return or the drive out which I assume is for going into an amps input..
http://www.youtube.com/watch?v=sQg4RZMtUXI
Few other demos on youtube too...
Other option is to pick up a 6505+112 used possibly do a head mod then switch between it and your riviera...