Yes tootler you are right - oops. But natural vibration frequency of a body, the return force of which is a spring (like a guitar string or indeed the top of a guitar IS dependent on mass.

That rules out going for one of the electric guitar tune-o-matics I linked to - my movable assembly is lighter than those, being hollow (those are solid).

I think I will have to go one of two routes - one being a cradle that holds a plastic or bone saddle (with an undersaddle in it). I have something like that in one of my mandolins and it is definitely loud but the problem has been getting the EQ right.

The other would be to retain the movable assembly but try first a different pre-amp (very high impedance) and second other stick-ons. I can test the first idea by using an old valve PA head I have - HUGE input impedance (or maybe a couple of belt packs I have - I think they are 1 meg input impedance, might be 3) but most stick-ons sound wholly vile and I am not paying out K&K money just to test a theory.