I spent today listening to the HE-560 with my iPad Air headphone out at 80% and they were actually fairly good, although they sound less energetic and impact is a bit less than with a small portable amp.
I've been telling people that they need about 1-2 watts of power to get the most out of them, but they really aren't bad at all when used unamped or with <500mw portable amps. However they won't be nearly as impressive sounding as they are with even a little more powerful amp. I told one person that my CEntrance HiFi-M8 with about 650mw at 35 ohms (estimate by another forum member) would be enough to keep me happy at a bare minimum. But others could likely be happy with less, based on my experience today.
Fang is having a a new production HE-560 shipped to me this week, and he tells me that the bass is even better now with jerg's baffle mod. I do have to ship my HE-560 back tomorrow, but the new set will be here soon. I'll have more to post after that, but I will certainly miss them for the few days they'll be gone.