I just got my HE-400 and I gotta say, these are probably the most sibilant cans I've ever listened to. Ear piercing highs... Every song I've tried (flac/320, various different artists and recordings, etc.) that has a lyric with the letter "s" in it, it's ear piercing. Cymbals aren't quite as bad but still painful. I've tried a lot of EQ in foobar to reduce the highs but I can't seem to get rid of the sibilance without losing the highs all together... and volume definitely isn't the issue
I'm hoping someone has a really really good plan for me?? It seems most people have zero issues in this category :(
I can comment that coming from the PortaPros I was using regularly prior to getting my HE-400, they were very noticeably brighter and harsher overall in the treble regions (the spike most mention being the worst of it causing some sibilance) - pads made little difference and were more of a comfort preference to me. Over time, I became more accustomed to their presentation - call it brain burn-in if you will, but I now enjoy them a lot for most of my music. They have a unique tuning along with having that 'American Sound', aka emphasis on the lower and upper regions. I knew this going in as I wanted a more fun, aggressive signature. It is possible to tame the treble with EQ for those who would prefer it less intense (as I've done myself) but I find lately I like them best with no EQ. I think someone that already prefers a presentation that is closer to neutral could easily be put off by these, with an equal possibility that someone who already prefers a brighter signature may be more comfortable with them. I've seen a few instances of headfiers that already own brighter headphones finding these less harsh (another V-shaped headphone, the DT-990, comes to mind). I suspect that the opposite happens to people coming from something darker, more bass oriented or with a softer, rolled-off treble.
There's a lot to take into consideration like your upstream equipment, music preferences and ear sensitivity, but I think their signature is different to the point where you have to listen to them pretty exclusively for some time to adjust to the sound. That's how it seemed to be for me, anyway. If you can't bear the treble presentation, these may not be for you - it's just an intrinsic part of their tuning. I have yet to try some other planars of interest (I'm pretty much sold on the technology at this point) or just other cans with different sound signatures, so may find I prefer a different tuning/presentation all together, and it may be the same for you. If you want to stick with planars, the Mad Dog or HE-500 may be more suitable options...I know I plan on getting the MD soon as a complementary to my HE-400.