gugman
1000+ Head-Fier
The only amp that gave me a "WOW" moment so far has been the BH Crack with the HD-800, I was using a Ember and Lyr2 when I got the HD-800 and it sounded flat without any dynamics.
For the HE-400i almost any decent amp like the Lyr2/Ember/HA-501 will give you great sound. I was able to try the HE-400i at my friends house with a AudioGD Master 7 and Cavalli Liquid Gold / Chord Hugo TT, there was a very small difference in the soundstage and the bass went a little lower but at price of those 3 components you are really in the area of diminishing returns. The HE-560 did benefit a lot more than the HE-400i with the upgraded gear but IMO its to far in to the diminishing returns also, I'd rather get a few more different headphones and more music to enjoy.
"Wow" more as an all arounder amp, all my headphones sound so much better with it, i did not mean only he400i