Surely the argument re values of Dukes rising faster is much down to the sheer numbers of 400/4's about. I have 2 400's so not knocking them. Had a Duke too for that matter and very agricultural compared to any Honda / Jap bike. Many see this as character, which again adds to demand!! My 500/4 I had back in the day, had more character than the 400, which is just soooo refined.
Tim