This thread is information overload to the nth degree. Sorry for adding to the cacophony of opinion and experience.
I have only had my HE-400s for a few days so it really is too early to say for sure how I will feel about these cans, but my first impressions are not particularly positive. I am using mine with the velour pads. I thought the leather pads were unlistenable
I have read, and read some more, and then read even more, and probably read some of the same stuff over and over again about the HE-400 and HE-500. I'm not new to headphones, and have owned at one time or another the following: Sennheiser HD650, AKG-K701, AKG-K1000, Grado RS1, and Grado GS1000. But I have not been a serious headphone listener for three or four years. I am back to needing a good sounding set of phones for serious listening late at night. The HE-400s are a lot darker than I was expecting, and they remind me a lot of the Sennheiser HD650s. At the moment I am driving them with a Woo Audio WA2 since that is the preamp in my speaker setup. I was planning on buying a WA6SE as a dedicated headphone amp, but that purchase was depending a great deal on how I felt about the HE-400. I wanted to wade into the Hifiman sound with the entry level planar magnetics first, expecting to also buy a pair of HE-500s to go with the WA6SE. Now I am not so sure. At the moment I don't even think the HE-400s sound as good as my Sennheiser HD598s, which cost half the price. The HE-400s are dark, they distort at high volume, and the treble is pretty abysmal (cymbals and hi-hats sound very tizzy and flat, with a pretty rapid decay). The midrange, mid-bass, and bass are all quite good even though they are dark. I don't know to what extent the HE-500s will address the shortcomings I am perceiving in the HE-400s and I can't tell from reading reviews and owner comments or comparisons.
For now I am willing to give it some time with the HE-400s. I'm not in a rush. But if I am not feeling better about them after a time then I will probably scuttle my plans, forget the WA6SE, and just buy a pair of Senneheiser HD600s and call it a day, saving myself a lot of money. Or maybe step up and give the Sennheiser HD800s a try.