According to my HD598 box, the spl is stated as being 112 dB @ 1kHz and 1Vrms. However in several places including a review, I see the spl rated as 112 dB /mW for the HD598. Obviously they can't both be correct with the nominal 1kHz impedance being ~ 50ohm. If 112 dB/1Vrms is correct that...

You can really notice the E12s uneven frequency response, with a slight grade at ~ 1 kHz. Measurements are with 33 ohm loads and with line-in level ~ 1.0Vp and gain adjusted to get ~ 2.5Vpeak headphone output:   http://www.jensign.com/FiiOE17/RMAA/CompareE17E12/Compare_E17_E12.htm

Some RMAA audio performance measurements comparing line-in, optical S/PDIF and USB modes using headphone loading of 33 ohm:   http://www.jensign.com/FiiOE17/RMAA/ComparisonFiiOInputs.htm Results are discussed at bottom of this page:   http://www.jensign.com/FiiOE17