There's no difference in sound if you run either pedal at 9V or 12V. Both pedals have a voltage regulator which smooths the voltage out and reduces it to 5V, then there's an inverter which produces a voltage of -5V. So total headroom of 10V, which works out to be around +10dBu (the exact specs are on our website). So with either 9V or 12V, you've got tons of headroom!