[jonfields45]

An EE's view on this...

The output impedance and the input impedance to which it is connected should be equal for maximum power transfer and highest signal to noise ratio. Line signals through balanced cables follow this design goal.

In the real world of passive signal sources, piezo, magnetic, dynamic microphone, etc. the source impedance is not resistive, but has all sorts of parasitics. A complex impedance consisting of resistance, capacitance and inductance is a function of frequency and a simple single number is not a realistic expectation. Often when you are quoted a number for a mag PUP, you are only being quoted the resistance component. For magnetic pickups it is series inductance which into the wrong load acts as a low pass filter. For piezos it is a double problem of series capacitance (high pass) and parallel capacitance (low pass). For mag PUPs you generally want that 10x (the source impedance) load, or higher across the frequency range of interest, to avoid that low pass filter. For piezos you have a double bound, too low a load impedance and you get a high pass filter, and too high you get a strange sounding low pass filter. This will depend on the construction of the piezo. It is hard to get that too high an impedance input problem with a piezo, but it can happen. Generally a well designed input circuit will be close enough to a simple resistive model.

Seem complex? With liberal return policies it is best to just try it out.