Sorry for delay. Contoured type, original, has 3-4dBs gentle lift at the low end (string fundamental frequency area), same at high end, presence > sizzle spectrum. This was designed in at the time when rigs were nowhere as good as they are now, and gave a slight lift to the sound when switched in.
Had a number of requests for non-contour version over recent years and as it got more known about with increasing numbers of requests, so we made it a standard unit. The flat unit has more EQ boost levels so that at max it matches the contour unit when the EQ boost and contour are added together.
Both types can achieve the same similar responses but some players, those that like more of a bridge pickup sound, were finding they were dialling out the pre-shape contour.
So it's more about the starting point, if you like more of an old school sound, James Jamerson or fat low end with sizzle, ie Marcus Miller style, then the contour works more immediately. If you like Jaco, Gary Willis style - more bridge pickup accent, then the flat response unit is a better starting point. If we redesigned it, probably include a way to have both.
