potterma
500+ Head-Fier
From the specs I've seen, a fairly astonishingly powerful one. I don't have much knowledge off the top of my head about which specific headphone amps measure well at very high output, though if the specs are accurate, the AMB B22 should be more than adequate, as should the Schiit Asgard 2. I can't unreservedly recommend either of these, since I haven't seen a complete set of measurements on either, but if the designs are well done, they should both be able to power an HE-6 just fine. An O2 should also be able to power the HE6 if (and only if) you listen to your music at a low enough level that the loudest volume spikes don't exceed the capability of the amp. If the published specs for the HE-6 are accurate, this should be true for the majority of listeners, but if Jude's measurements on the HE-6 are accurate (which imply a significantly lower sensitivity than the manufacturer's spec), the O2 becomes a much more questionable proposition.
(In fact, given the power demands of the HE-6, it might make sense to use an inexpensive speaker amp to drive them, since many of the headphone amps capable of doing the job are relatively expensive anyways).
While from their respective spec sheets the Asgard 2 should be able to sufficiently drive the HE-6, it falls solidly on its face when compared to a decent speaker amp.
And it turns out, there is a lot of difference between "being able to power the HE-6 just fine" and doing an amazing job of driving the HE-6. For most of us, a "price no object" headphone amp is just out of the question. However, with careful selection of a quality speaker amp, one can truly experience "summit-fi" with the HE-6.