We're a tiny startup trying to take advantage of this, I'm just a contract programmer though and it's more exploiting licensing costs than anything. I don't really disagree with you.
Songza's headphone-specific equalizer made my Sennheiser HD 558 on iPhone much more enjoyable with the profile for HD 580. Hope they provide a profile for HD 558. Hardware headphone amplifiers are obsolete. Software equalizers are the solution.
For many mobile sources, you'll really want one for 250 Ohms. The HD558s by comparison are really low impedance.
An EQ can certainly help make your headphones sound more "interesting" or "better" though.
Huh? Equalization and amplification are rather orthogonal concerns.
What?
My headphones need a lot more power than is provided by a typical headphone jack. Not to mention the DAC built into most amps that gets rid of any line noise. Equalizers and amps are two completely different things, too.