I realise that I'm cutting at below the bottom frequency of the bass. What I wasn't sure of was whether that was the frequency at which the cut starts, or whether it was the -6dB frequency, or what, and whether any other artefacts might be introduced, so experimented to see if there was any discernible change to the sound. I know what the Thumpinator does, and that's why I thought it would be an idea to put an HPF in my effects line, and why I'd like to eventually build a Sallen-Key HPF (which would actually be two Sallen-Keys, giving 24dB/octave roll-off).