Here’s a quick ‘n’ dirty methodology I use for leveling the volume of my headphones:
- 70% of Windows volume – always controlled by keyboard;
- Launch a white noise WAV file I have and adjust the gain/volume knobs in my amps until 85 dBA is achieved.
You could use a decibel meter app for the above. After all, you just need a reference. Taking one example from your previous post:
At 2’, close to the solo part, my Windows volume was at 40% with the SRH1540. Higher than that, it not only gets uncomfortable but the Shure itself starts to distort to the point there’s a spaghetti of frequencies. Not sure if this is the song, my DAC, the Shure itself, or a combination of all three. The meter was spiking 83 dB for the micro dynamics.
Comfortable listening level was 30% for that particular song. But remember, this was to my system. Everyone’s reference is different. Same deal with HD600 (more mid forward / neutral). 30% was OK to me.
That was a very aggressive song by the way, melody wise. It’s heavy metal, right? 
