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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.

I can't say I've ever noticed any humming with my Mark III Poseidons. My Klipsch Chorus are quite a bit on the sensitive side, so I'd expect I should notice any problems. I dunno, maybe I just got lucky? Looking at my photos, it appears that C is connected to the lug where the high voltage center tap lands. You can see my wiring snake around the left side of the photo, then across the bottom. I don't recall giving the matter any particular thought when I wired it up.

Could you put your ear into the woofer cone when the room is quiet to see if you notice any hum? I am curious if you have that faint amount of hum too.

The rule of thumb has always been to ground the HV transformer
center tap and the negative lead of the first capacitor together
directly, no space between (which would be the can in a Dynaco)