Holding my hand up - played a local fete this weekend, and the sound engineer plugged the bass directly into his DI box, then I took a feed from the parallel output into my pedalboard, then to my amp.
Turns out that as soon as I turned on my recently finished modded FX25 envelope filter I overloaded his system.
I was playing about with a trimpot on the input stage of the pedal (to set the 'sensitivity' sweep of the knob properly) and only used a 25kΩ pot - I suspect that the input impedance wasn't enough, and that's where the problem was.
Time to strap a 1MΩ across the input to ground then.