okay, here's something i ran across:
9 Volts vs. 18 Volts
Most active pickups and preamps operate on at least one 9 volt battery, and some use two wired in series, delivering 18 volts. 9 volts works fine for many instruments, so what’s the difference? Well, there are two kinds of 18 volt systems: 9 volt systems that can handle 18 volts, and systems designed for 18 volts.
Many 9-volt systems, like EMG’s, will operate at 9, 18, or 27 volts, and can be supplied from phantom power in a three-conductor cable rather than an internal battery. These systems don’t operate much differently at higher voltages but gain a little better transient response and headroom for percussive signal peaks when run at 18 volts. Any performance improvement between 18 and 27 volts (two and three 9-volt batteries!) is generally considered to be insignificant.
Systems designed for 18 volts, like the Aguilar OBP-1 and some Bartolini preamps, utilize the additional voltage to increase both headroom and the amount of gain that can be provided for E.Q. The Aguilar circuit allows 18dB of clean bass boost, even when slapping a low "B" bass string. The Duncan Hot Livewire set for guitar uses 18 volts to deliver an output hot enough to send even the cleanest guitar amp into overdrive.
