Another observation is that one VAC 140 will pickup humm if paired with another amp on the same port.
Perhaps, along the lines of what I was suggesting, that is because with the two amps each drawing substantial current from the same port, the voltage offset between that port and the port that the preamp is on is increased. Causing increased ac current flow in the shields of the interconnects between preamp and power amps. Again, your problem might be too many dedicated ports for the interconnected components, not too few.
Regards,
-- Al