I just upgraded my MKIII's to a Poseidon Board. The first time I fired it up it hummed. I discovered it was caused by ground loop in the design of the Original MKIII's, when upgraded to a Poseidon.
The manual says eylet C is connected to the ground lug on the main filter cap. It should really be connected to the node the center tap of the high voltage winding is connected to. This eliminated the hum.
Is the manual incorrect or is it assuming a certain setup?
Also, there is a small amount of hum that is caused by the AC filaments in the driver stage due to filament to cathode leakage. It is very faint, and can probably only be detected on highly efficient speakers and a listener with a very good ear, like myself. Certain tubes have more leakage than others.