In reply to:
I suspect that some headphones are harder to drive than others, and it isn't just a mV output thing. Could some speakers be harder to drive than others, and it's not just an all-or-nothing thing? (I.e. the B&Ws sounding bad at all volumes on an underpowered solid state amp).
Sensitivity varies widely between models of headphones - ask any ENG shooter - the HP jack on an ENG one-piece is so underpowered that once a shooter finds a brand of earbuds that he can hear over traffic noise, he'll stick with them until they quit making them... also good people to know when they're trying new models - if they're not sensitive enough, they usually throw them at the closest person - I try to be him.
Bren R.
