Just checked the datasheets for a bs170 & a zener 9.1v diode. If I'm reading them correctly, they can both take 18v which leads me to believe a SHO circuit can be run @ 18v provided the electro caps are rated high enuf. Are my mind-grapes working correctly on this?
As far as I know, yes, but there's no point. The extra voltage won't be transformed into a better sound for you. It might give you more headroom, but there's no added by doing that that would benefit as far as I can tell.
I'm thinking of making a 2-in-1 with an OD capable of running @ 18v & a SHO for either a pre or post boost.