I think your understanding is fine: these measurements
do represent the "worst case" (highest/loudest) signal my headphones would receive for a given dac/amp volume setting.
I just didn't do a good job explicitly interpreting the consequences here
Example 1: if the song I'm listening to has an RMS level of -10dB (peaks at 0dB, though), and my headphones produce 100dB at a given dac/amp output, then the average intensity hitting my ears is 90dB.
Example 2: for HD-6xx sensitivity of 103 dB/V, listening to a track of -10dB RMS, the volume of the dac/amp set to 0.45Vrms, would result in an average volume of 103dB + 20log(0.45) - 10dB = (103 - 7 - 10)dB = 86dB.
Unless I'm out to lunch here... could be... first cup of coffee has not yet taken effect