The problem is not the CJ amplifier, it's the extra ground. The earth is a poor conductor and that results in steady state potential in the ground -- which is the source of your humming.
An extra ground is not a good idea. Lightning surges isn't the only problem -- transient spikes occur all the time, such as utilities switching capacitor banks and on-site generating facilities switching loads from utility to generators, any of which can produce transients to fry your electronics connected on a grounding system with a potential across it.
An extra ground is not a good idea. Lightning surges isn't the only problem -- transient spikes occur all the time, such as utilities switching capacitor banks and on-site generating facilities switching loads from utility to generators, any of which can produce transients to fry your electronics connected on a grounding system with a potential across it.