I did the same with my Ashdown acoustic pre-amp pedal this weekend. Think I've managed to suss out the notch filter on it now (ranges between 60 and 300Hz on a rotary control, but no graded markings around it to give a rough idea of where you are on it!).
Was getting some feedback at the gig I did on Saturday evening with my Uke basses - on the subsequent recording it looked to be about 227Hz.......