Well, Maj, it may be your exceptionally high personal magnetism(or electricity), or when you touch the input it's then grounded through your body and a ground loop is created. It'd be remarkable if the ground loop current was high enough to create a driver movement that could even be felt(unless you meant with your finger), though. Does the chassis ground screw(if the 800 has one)make a difference? No brilliant ideas; let Axiom check it.

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Enjoy the music, not the equipment.