I can only give my theory, which is based on the apparent fact that this noise issue is somewhat new - the incidents as reported on the forum have mostly been in the last year or two - and sometimes a new tube will seem to resolve the problem.
The heater winding is grounded at the center tap already, which allows it to serve as an electrostatic shield between the primary and the high voltage secondary. But the filament voltage is only connected to the winding for the short intervals when the rectifier bridge is conducting. Most of the time it is floating, and if there is a lot of leakage current between the heater and the cathode, that would cause the common-mode voltage to drift until the next connection, which happens 120 times per second. So my theory is that, as the supply of 6DN7s is drying up, more of them have excess leakage. Lots of suppliers cherry-pick the good ones and return the bad ones to the marketplace. The artificial center tap on the output of the power supply drains the leakage current to ground.
Paul,
I just installed the resistors and it works quiet well. Not hearing any downside so far
