I'm DISAPPOINTED I didn't learn this sooner...

[@b-spot]

It depends on the circuit. If it's something that uses clipping diodes like a tube screamer, sd1, or mxr distortion plus, the effects will be minimal, as it doesn't change the behaviour of the diodes, only the transistor/opamp/jfet stages. However if you were to try this on something like a colorsound power boost, tc inegrated preamp, or many fuzzes, you'll notice a much more significant change in headroom and tone. The change in supply voltage increases the potential voltage swing of the gain stages. I should note that this won't have much effect on pedals with built in voltage regulators and should not be attempted with pedals that have internal voltage boosts/charge pumps, etc. as it can damage the pedal.

[@mastercraft5798]

Listening through my good quality stereo set up - I couldn't hear any difference. Maybe at volume through a guitar amp it would be more audible? Who knows, but I guess not.

[@Bradjohnsonmusic]

Yeah, I didn't hear a discernible difference to warrant going out of your way to power the drives with 18v. It reminds me of when mixing music and thinking that the plugin you just put on is magic...only to realize it's not turned on :)