This is similar to what I'm working on with a guy right now, with the difference being that he's got a Suitcase. The static crackle is heard when he plays and is even worse when he hits the sustain pedal! He says it's one of those things where everything was fine, then he moved the piano a few hundred miles and weird stuff happened.
check:
harness
solder joints
and the things mentioned above.
when working with a suitcase its always nice to break it down to:
is it the top, the cable or the amp causing the problem.
so, check the harp's output with another amp, preamp, mixing desk what ever
use the amp with a guitar or IPOD (turn the volume down) in case it has the power amp input jacks.
check the preamps outputs with a mixing desk/hifi amp in case the suitcase amp has the preamp output jacks.
kind regards
Mark II