Won't that hurt them?

No, not at all, just make sure that the polarity is correct... meaning that the center pin gets the Negative (-) just like the 9volt adapter you'd use with it.

What will it sound like?

Quite different actually.... the FD2 gets much more dynamic, a little brighter and a lot less compressed, more articulate, more amp-like, with more of the strings coming through... very nice!. The "Comp-Cut" mode gets a serious increase in headroom without distorting. Using 18volts DC, the FatBoost gets tons more clean headroom, doesn't distort at all, and can get upwards of 30db of crystal clear gain. The Bassdrive just MUCH better for Bass guitar with the extra headroom and clarity.

WARNING !

Do NOT try this on any other pedals by any other manufacturers without first determining whether they have components with high enough tolerances or the result may be fire and/or serious injury to you and perhaps your pedal. The FD2, Bassdrive, and Fat Boost are over-engineered to withstand the higher Voltage.

Here's an easy way to get 18 volts off of 2 x 9 volt Batteries!

Here are instructions for an 18 volt cable you can make ONLY for use with Voodoo Labs Pedal Power OR 2 separate 9 volt DC wallwart transformers! Warning!: Using this cable with a Dunlop Brick, Furman, One Spot, or any other NON-DISCRETE power supply will result in damage to the power supply and/or to the Pedal!