You're using those transducers, right? Did you check that their output, and the effect on the pickup of that output, is linear?
I haven't analyzed pickups in isolation yet, but in the past I have recorded the instrument's output into Audacity, then selected a portion of that a few seconds long, then with Menu ...Analyze ...Plot Spectrum (set "Size" as large as possible (this improves resolution), I was able to isolate the tone=0 peak of which I spoke (around 300Hz), easily if I remember right. EDIT: I did that just out of curiosity, not a serious test.