I am so confused about this one. I have spoken to 2 senior guitar techies about connecting another 9v battery to my 9volt bass with EMG P/J pickups. They said "not a good idea might cause overload" and another "Its not designed for that purpose". Reading from the posts on TalkBass I thought that was a good idea.So many guys said it would give more headroom and tone.
I recently had a EMG J set installed in my Warwick Thumb. The instructions that came with the set advised that two batteries wired in series for 18 volts is recommended for bass to increase headroom and enhance overall performance. I asked the guitar tech to wire it for 18 volts so I don't know if there was any difference with just 9 volts