I should have limited my comments to mobile listeners with mid to low impedance headphones (which are the vast majority). I used to listen to iPhone with HD 580 (300 ohm) directly, with full volume and no amplifier/equalizer. It's loud enough for sensitive ears, and very enjoyable, better than HD 558. With amplifiers the sound is a little better, but not worth the inconvenience. For mid to low impedance headphones, amplifiers are often used to improve sound quality, not volume. Software equalizers (like those in Songza) would work well, and further shrink the market of amplifiers. Remember that in this mobile internet age, the American consumers have voted for Beats, I'm not sure what for, perhaps the overall user experience.