Recently acquired a transformed coupled headphone amp which was custom built to my specifications. Its been fantastic so far and synergises well with most of my headphones. One thing I recently discovered though is if I any hyper sensitive headphones off it like my 25ohm 108db/mw Denon D7000 or IEMS I can hear a tiny bit of 50hz mains hum when no music is playing. Now whats interesting is this when I increase the volume to 100% the level of the hum doesn’t increase at all leading me to believe its the tube heaters themselves rather than the transformer noise. It is not audible with all my other less efficient headphones. Curious to hear what your opinions are and whether or not this is normal? or what you think the cause is? (Its not a ground loop btw this happens when nothing is connected to it).