OK, thanks for that; I understand better what the problem is. Jack Casady is at the top of my favourite bass players, and, one day, I'll be able to afford his signature Epiphone. It has a rather unique pick-up set-up, with an impedance-changer for getting different tones with different amps and consoles. I would guess that the impedance of the bass is not working well with the impedance of the pedal. Does this effect change when rotating the impedance control, or is it at every setting..?
My 'go-to' bass is a Hofner Verithin, as acoustic an electric bass as it's possible to find, and it is very delicate when it comes to feedback. Personally, I like this, as I can approach the cab (Fender Bassman valve amp into Fender Bassman 2x15...) and 'play' the resulting tones. It's true that I never use pedals, so I've never used an OD signal; I would imagine that, being hollow, it would howl like a banshee at the slightest provocation. We live isolated in the countryside, so I'll try it out to see what happens, though. If you open a window, you might be able to hear it from where you are..! I'll report back...