Can I use multiple batteries?
Yes. If you’ve got room for multiple batteries in your guitar, you can use two batteries
wired in series to power your onboard circuitry at 18 volts. The output level will not
appreciably increase, but you’ll have increased headroom and crisper transients. This
is especially useful for percussive/slap bass styles where you can generate enormous
instantaneous power levels across the entire frequency spectrum.
You can also wire two batteries in parallel to provide a regular 9 volt supply but with
much longer lifespan between battery changes.
Although most of our products are rated for 27 volts, we recommend a maximum of 18
volts. The additional benefi ts of 27 vs. 18 volts are negligible.
http://www.emgpickups.com/pdfs/faq/EMG-FAQ-Battery-Questions.pdf... и добавил:да инфы то везде куча, просто хотелось получить объективный отзыв от тех кто пользует 18В...