Here's my take on what makes a Preamp Pedal different from a Distortion "effect" pedal. It all comes down to what the intent was of the circuit's designer and how they tuned it for each specific use.
03:09 - AMT P2 Drive Out into Front of Amp
06:17 - AMT P2 Drive Out into Power Section
07:25 - AMT P2 Preamp Out into Power Section
10:08 - Gristle King into Front of Amp
13:05 - Gristle King into Power Section
14:03 - Xotic BB into Front of Amp
16:44 - Xotic BB into Power Section