They're actually easily driven by the iPod, which surprised me (I didn't initially intend to use them for portable listening too).
Also, I still don't understand why people quote such a high impedance for the HD 555 whilst the sennheiser site has always rated them at a 50 ohm nominal impedance.
As for outside noise, it's not that bad, really

I definitely notice a difference on a 'regular' not-so-special amp at home, even when there's [some] noise going on (obviously you won't be able to listen to open cans very well in the first place when there's a whole lot going on, but you *do* notice a difference amplified

)
Volume is, surprisedly enough, very acceptable still..I keep them at anywhere between half-volume and about 90-95% preferably.
That's pretty good considering most people use to the ipod's standard shitty earphones on full volume ;p (hi2u tinnitus)
ps: The
Headroom Airhead is 2.875" (7.3 cm) wide, 1" (2.5 cm) high, and 4.125" (10.5 cm) long, and weighs 'just under' a pound with batteries.
(With the deluxe version the '
Total Airhead' measuring about the same but built for an even better performance)
Still.. it may be an awesome feat to put so much into such a small amp: that's still a bit big for me! 10,5x7,3x2,5 while my ipod mini is already at 9,2x5,1x1,3.
Sure, I'm used to carrying big packages in my trousers (
LOLOL), but it's still a lot to add to it all.
As I said though, it's a dillemma

and it might be worth it, haven't made up my mind

the money is definitely a factor keeping me from deciding anything in the first place, though.