[fetteler]

I don't know about centering the bands on each string as the notes of the open strings are only a small fraction of the notes that will be played. Also there is a huge percussive element to the banjo sound and the frequencies that you would tweak to adjust that component of the overall timbre are probably fairly unrelated to the open string notes.

Having a play with a graphic eaqualiser is a good idea to start with but it's probably even more useful to have a go with a preamp/equaliser designed for a stringed instrument like a guitar. Lots of these pedal type accessories are available inexpensively and you may find that they are all you need and there's no need to re invent the wheel so to speak.

Steve.