It has occurred to me that I am very unlikely to ever use the Jazz setting, so I'd like to dispense with the switch and maybe use it to switch in a mid cut to make it more like a Super Reverb. I know this might be sacrilege to D* adherents, but what the heck!!
Problem is I don't really know where to start. I've downloaded the Duncan Amps Tone stack calculator
http://www.duncanamps.com/tsc/ and created a profile for the AB763 and OTS circuits as best I can and I can see that the OTS is almost flat whilst the AB763 has a big dip at about 700-800Hz.
If anyone has used this tool as a basis to do mods I appreciate them chiming in with suggestions for component values.