[Grobbendonk]

What you can try to do is to send audio into the spring tank via the Speaker Output, as it has the most "juice" in it. Since it normally moves a speaker it might potentially also move the springs of a spring tank. And then you can take a contact microphone and just sort of stick it into one of the springs on the other end of the sound input. This way you should be able to physically pick up the sound again at that end.

So, in theory, that could be a sort of "poor man's spring reverb" :D Please try that and let us know your results!

that sounds adventurous ( :  i'll hunt for the hardware and try that - thank you!