I'm most pleased with my new Grado PS500 open on-ear headphones driven by either my iPod Classic (7th gen) or my iPod Touch (5th gen). Specifically, I get enough volume in the headphones with the iPod volume set at about 60%.
So... besides volume, what other advantages are there to adding an amplifier? Do I get faster response to transients? More transparency? I'm considering adding an amp, but only if sound quality can improve even further.
(I had tried the Fiio E6 amp, with its .125W into 32 ohms, with my Sennheiser HD 598s... other than volume, I could not distinguish any improvement in sound quality, suggesting that that amp was not helping those headphones. I realize the E6 is a very small amp... for my Grados I'd be looking at a Schiit Magna or a Fiio E12 Mont Blanc, both of which are over 0.800 W into 32 ohms).