If you have the 3 band (i.e. Bass, Mid-range, and Treble) pre-amp, you're good. I wouldn't change a thing unless there was an issue with it. Otherwise, the only difference between the 9V and 18V Pre-amp. are the number of batteries (1 vs. 2) and the Dip Switches. Your not getting more output, you are only getting more battery longevity. The Dip Switches in the 18V provide frequency adjust flexibility, however, I find no need to adjust from Ken's factory settings (old vs. new).
Maybe some day I'll spend more time with different DIP Switch settings (doubtful though). If I need to adjust between 20Hz and 12k, I have all the flexibility from the bass and amp EQ: Not a problem.
