They're website just says "Sensitivity: 103dB".
Knowing HiFiMan, and noting else about that model, it could just as easily be 103dB per
watt.
"Sensitivity" is usually specified in dB per volt, but that's not really all that high, especially for only 25 ohms...
They could mean efficiency instead, which is usually dB per milliwatt, which actually wound make it easy to drive for a planar...
Rtings seems to measure everything but efficiency, so idk what it actually is though. Anyway, at 25 ohms, here's the difference between 103dB/mW on top and 103dB/V on the bottom.