I.e. current limiting resistors to help with turn on surge for rectifier long life
On 100dB speakers anything small is audible. Easier is apply output FFT to watch harmonic noise spikes and experiment. At least monitor on a scope trying to achieve cleanest output line on max vertical sensitivity while without signal. Or worse case use a good DMM set on AC mV. Try alternative wiring schemes to read less mV on idle. Since your implementation is personal regarding PSUs and housekeeping decisions can't be sure, but I would first compare heater supply lift vs grounding approach.
10dB line gain on 100dB speakers ought to bring out some of that too. But it differs between tubes. The 6Π6C has bit more than some others in my experience but not that much.
To elevate the heaters above cathode potential to prevent potential noise leaking to the cathode. MJ added this circuit to lower psu noise I think. But it is debatable if it make a difference at all. I just used it because it was on the board. But still will try as Salas suggested that I might just ground the DC instead of lifting.
My system is plain old typical inefficient, ProAc speakers. I hear a slight trace of noise, what I'm guessing is thermionic noise from the tubes, I really have to press my ear right up against the drivers to hear it. Not a problem for me. But would there be a benefit to placing the volume pot after the output caps? Our would that mess up the feedback?