I don't claim to know everything or more than headphone experts, but I know the BASICS of what can physically make good sound to realistic, uncompressed levels, also I'm an electrical engineering major, obscessed with the inner-workings of loudspeakers. Don't give me theoretical spl numbers and curves that assume the manufacturer rates SPL accurately. None do otherwise a 15-20mW portable would drive nearly any low impedance cans louder than an ear bleeding rock concert on those 100+db/mW headphones, in fact that portable would drive 100+db 600 ohm pair LOUDLY if that were true and the bass on almost headphone no matter how cheap BASED ON SPECS would SOUND (I didn't say feel) like a huge, low FS subwoofer! I've heard subs that hurt and make ears ring fast and they were only 88db/1w, but they had displacement, lots of it. I KNOW nobody can tell me a "gutless" portable can drive ANY existing headphone to that of a loud concert WITH simular or even more monstrously loud bass if the bass were also that efficient and deep. NO spl and response specs are all marketing, NOBODY HAS EVEN BOTHERED TRYING proving it otherwise, and I can't believe how many people actually believe such BS.
Everyone, Efficiency IS NOT RELEVANT in sub-bass reproduction. May be in mids and highs or mid-bass, not sub-bass. JL Audio tutorials explains this quite well, even though they do make car audio gear where overblown specs are rampant, they are right about this. Displacement capability and voice coil overhang (linear excursion) determine this, not spl. Actual reference SPLs are based on Fs, Qes, and Vas, and no headphone driver coupled or not will make 100+db at 1mW with frequencies 20-40Hz or every teenager who bumps his boomy car subs at well over 120dbs would have a pair of headphones too that could do the same, however NO headphone does 120db in these frequencies because its physically impossible from 4-5cm over-ear drivers. Estimated effective dome areas: Sennheisers: 0.58in2 SD, V-600/Grado 60/80: 0.95in2, V-700: 1.4in2. Clearly why Sennheisers take minimal low frequency power to overdrive, they have no more x-max than the others and even if they did the highs would still suffer from the reactance changes due to the huge coil overhang outside the gap as well as huge physical excursions. More area is what is most important. The V-700s have enough, but apparently nobody else has taken a look at their headphone drivers and wondered or I would get helpful advice as oppose to flames and efficiency misconceptions. If someone wants to give me SPL numbers, let it be ones they measured themselves and frequencies measured as well or its a useless number.