I'm really confused about my headphones' specs. My Grado GH4's impedance is 32 Ohms with 99.8db/mW sensitivity and my MM500 is 18 Ohms with 100db/mW. I tried in many equipments even with my iPhones, in the same volume displayed on the equipments, the GH4 is always much louder than my MM500, and RS1X I tried too is also louder than MM500 in the same way of GH4's with same specs of GH4. WTF. Is there some kind of higher theories in physics that I miss or have to know? *Edit: wrongly write "mV" in a hurry on the post, but it should be mW.