I think the 9V vs 18V is just a personal preference kinda thing, anybody could like either setting more with any instrument, I think. I think it makes sense that a bass signal and tone would work great at 18V. For me though, I like the really saturated distortion kind of sound so 9V is better.
I think it also depends on how clean your amp is too... my amps are totally clean so I need the pedal to be as gainy as it can, as distorted/saturated as possible. But with an amp that's cooking a bit 18V might be better, more transparent, might make the amp cook more without making it crazy distorted like it could at 9V.
I just always say try it all and see what you like best, there's no rules here man! *smile*