there are different ways of listening - for all day background music you do want to keep the average SPL way down
but it is a plausible "Audiophile" goal to want to reproduce the "live event sound" as a infrequent treat - say less than an hour long session once a week - which even for strictly acoustic music can involve dynamic peaks of >120 dB SPL to accurately reproduce symphonic peaks, big band jazz, hard hit drums, cymbals without transient clipping when the amp runs out of juice
for Senn's higher end 300 Ohm cans ordinary op amps ran at +/-15-18 V supplies just manage - for AKG, Beyer's 600 Ohm or low sensitivity "monitors" I would really like more V than most op amps can manage - and portable amps overwhelmingly use op amps
for portable use most could forgo the "live event" SPL requirement - but some users may be disappointed by single "9V" battery supplied SE amps with higher Z, insensitive cans - again few portables use V doubler circuitry to boost the "9 V" battery (which can be as low as 7.2 V for some "9 V" rechargables )
before you run out the inane, knee jerk "you'll go deaf at those levels" remarks try actually critically reading
http://gilmore2.chem.northwestern.edu/articles/hearing_art.htm and look at Moulton, Katz, Cordell or other serious sound pros for estimates of real event peak Dynamic (ie short time, transient) SPL - no one is suggesting average levels of 120 dB - just the ability to cover the Peaks without clipping
but yes many listeners raised on "Loudness War" victim dynamic range compressed drek with <10 dB peak to ave "dynamics" could be happy with amp/headphone combos that never reach 100 dB SPL
to mangle another industry's tag line "If it can't get loud enough to make you deaf it can't get loud enough to sound good"
it is possible to want a headphone/amp combo that can exceed 120 dB SPL for dynamic peaks and to carry broad-band attenuating ear plugs for amplified Club "music" and Rock Concerts which can easily have too high SPL for too long and even single night's exposure can leave erars ringing and cause permenant hearing loss