I finished fixing the heater situation in the DarkVoice this morning.
As I mentioned before, the heaters in the DarkVoice are not center-tapped, either at the transformer or virtually, which I suspected was a large contributor to the 60Hz noise issues numerous people are having with the amplifier.
Instead, what the designer did was ground one leg of the heaters at the 6SN7 socket here (it has been removed, but a connection was made between the 6SN7 heater and the front panel LED PCB ground connection).
So that connection was removed and a virtual center tap was made using two 100ohm resistors at the point where the heater windings hit the center PCB. The original wiring was removed and replaced with a twisted pair. I also mentioned that the tube heaters were being run over voltage, at around 7.15VAC. Just so happens I had the perfect voltage dropping resistors for the job on hand, two 130mOhm 5W wirewounds. One goes on each leg of the heaters since they are now balanced.
Now getting ~ 6.3VAC on the across the heaters (note that since the heaters are now balanced, each heater-to-ground voltage measurement will be 3.15VAC, the voltage from one heater to the other will be ~6.3VAC). Worth noting that the front panel LED is powered from the heater winding, the value of the resistor in series will need to be adjusted since the voltage has been halved, otherwise it won't light up.
So did balancing the heaters make a difference? Well, I have two 6SN7s on hand that were completely unusable in the DarkVoice due to heater-to-cathode leakage, the 60Hz noise was completely out of control. One was this black-plate Fivre 6SN7GT. I have had no problems with this tube in any other amplifier.
In terms of the hum level, the difference is massive, but there is still a low-level hum audible without music playing, took it from a 9 down to a 2. I suspect this is due to primary-to-secondary leakage in the mains transformer, which is not going to be fixed without completely replacing it, at which point you might as well just build a new amplifier. Regardless, I would still recommend this modification, center tap and balance the heaters and break the ground connection from the 6SN7 heater to the power LED ground. The hum could likely be further reduced by using a 500ohm trimpot with the wiper connected to ground instead of the two 100ohm resistors. The pot can then be adjusted to minimize the audible hum, it's called a humdinger.