"Some of them use an electronic bypass, so the OP simply could use a
stomp box to compare the sound of adjusted impedance with non adjusted
impedance."
I should explain this ;).
It's much likely close to a DI box when in bypass mode, so that no
effect affects the sound, as well as when using the stomp boxes effect.
To compare the signal, the OP needs to connect and disconnect the stomp
box to/from the signal chain ;).