adamus - If you can hear the noise/hum it's not likely coming from the SM heater circuit because it runs @ 150khz (unless your hearing is REALLY good
I think it is not uncommon in CTHs that, depending on various factors incl the tube used, a very faint
be audible with very efficient cans.... Between songs or when no signal is present.
If its more than that, I'd look at grounding (incl pot grounding) and would consider boxing it up & getting it away from all the nasty signals roaming around the bench.
P.S. Even though we incorp a ripple filter in the SM heater circuit, it is possible that some insignificant amount of ripple is still present in the heater supply. During prototyping, I couldn't tell battery-powered heater from this SM implementation, so think this should not be a factor. I bring this up because you should report the frequency & magnitude of that wave you are showing to assure its the audible-range one that you are hearing.