It is possible that this is as good as it gets. Stereomour was never designed to be a headphone amp, after all! To determine whether there are contributions from other causes than the filament supply, we need more data. There are two items I'm thinking of in particular:
* Frequency spectrum of the noise. Ripple on the filament power supply is 120Hz and harmonics; the hum pot would reduce the 120 to near zero leaving 240Hz dominant. If there's a way to get an FFT of the signal that would tell us. How did you determine it was 120Hz-dominant?
* Actual AC voltage level of the hum. If it's louder than other Stereomours with DC heaters, then there's probably another cause.
Both are somewhat tricky to measure. Few meters can read less than a millivolt AC with any accuracy, but you can measure the voltage across the primary of the output transformer and correct for turns ratio. Do you have access to an oscilloscope?
Even if it turns out to be performing to spec, there are still a couple things to try. PB suggested wiring the outputs for 4 ohms; you can even wire them for 2 ohm to get another 3dB noise reduction, at the price of not driving a speaker as powerfully. Or you can attenuate the output with an L-pad, which limits the maximum power available but reduces all upstream noises.
