Parker Fly Handbook page 31

There is only one safe way to rid yourself of pesky ground loops.
Use a direct box with a ground lift to isolate the two amps. Plug the magnetic
end of the stereo cord into one amp, and the piezo end into the direct box. Plug
the direct box's output into a balanced input on the second amp.
Eliminating Piezo Distortion
The piezo system has a far greater treble response than normal magnetic
pickups. Some amps will have no trouble with the additional high frequency
output, but others will distort. If you are getting distortion, you may want to
decrease the gain of the piezo pickup. The way to do this is to lower the piezo
volume ( see Balancing piezo / Magnetic output), and /or roll off the treble
on the amp.
Supplied Tools
3/32 T-handle hex wrench
5/32 hex wrench
T-27 torx wrench
Step-stop/fly wheel wrench
Stereo cable
.10 flat spring
Tremolo arm