Bought my son a pair of HE-400s for Christmas and ended up buying a pair for myself after the recent price reduction. For the most part these have been spectacular.
The only issue I have is that selecting an amp for portable/transportable use with my rockboxed iPod 5.5g has been way more work and cost than I ever imagined. Both my son and I have Dragonflys, which work well with the HE-400s, so I was expecting a similar experience with a portable amp. Much to my dismay it was not. Here are my brief impressions:
Dragonfly: Has adequate power and drives the HE-400 with pretty good authority. The bass/mids/highs are all well balanced. There is perhaps some congestion during very loud complex orchestral passages; however, overall the HE-400s sound good with the Dragonfly. Only problem is the need for a usb device.
Leckerton UHA-6S MKIIs (ADA46271B): Good neutral frequency balance which meshed well with HE-400s; quiet; and well constructed. Just didn't feel it quite had the power necessary to properly drive the HE-400s. If you listen at fairly moderate levels you might find it acceptable. Very nice amp that I really wanted to keep. Nick is also a pleasure to work with.
JDS 02: I know many drive the HE-400s with this amp but it just didn't work for me. Yes it's very powerful clean, and dead quiet but my impression was that it really exacerbated the potential for sibilance and harshness of the HE-400s. Bass wasn't as solid as I would expect from a powerful amp like the 02. Thought something must be wrong but subsequent bench testing at JDS proved otherwise. Just a really unlistenable combination for me. Your mileage may vary.
Headamp Pico Power: Wow. Perfect match for the HE-400. Ample power - using high gain. I rarely need to go above 12:00
which for most material is veryloud. Of course, it's not just about sound pressure levels, the Pico Power provides a sense of authoritative control and richness that brings out the best qualities of the HE-400s. It has powerful defined bass; neutral midrange; and sweet extended highs. It won't mask the treble peaks on the HE-400 but certainly doesn't draw attention to them. Everything just sounds "right" with this amp. Drawbacks: I worry over damaging this piece of art and my wallet is $475 lighter!
Bottom line is it shouldn't require this much investment to make the HE-400s sound their best. Granted, I needed a transportable amp which limited the options. Yes you can coax pretty good sound at moderate listening from most headphone outputs (regrettably not my particular 5.5g iPod) but you may need to experiment with a proper amp with plenty of juice to make the HE-400s really shine. Not sure I would have embarked on this journey if I knew what I know now. That said I am completely satisfied with this setup and want for nothing more.....for now. :)