Methinks you have no way to tell, accurately, the SPL at your ear!
Fair enough!
Right! The DAC knows NOTHING about the SPL level. That's why I said "not calibrated".
Some amplifiers have more gain than others and different headphones/speakers are more sensitive than others and with speakers it depends on how far you are from the speakers, plus almost everything has a (analog) volume control.
With speakers you can use an
SPL meter but they are not accurate with headphones.
With headphones it's almost impossible.* You'd have to measure the voltage and make a calculation based on the headphone specs,
but regular multimeters are designed for constant voltage so it only works with constant tones and you can't get an accurate/useful reading with music. With an oscilloscope you can measure the peaks but not the average or RMS so that's nor useful with music either. And then there's
A-weighting which is standard with SPL meters but doesn't exist on multimeters or 'scopes.
* There are special calibrated headphone-measurement setups, and Amir can do it.