A few questions @ solderdude and others with experience:
(2) Have you, as I asked Amir, compared these with the HD6XX? What are your impressions and/or explanations of differences beyond FR?
I own the HE-400se and the 6xx, so I'll try to compare across the spectrum:
SOUNDSTAGE
-- Neither is very wide. The 6xx definitely feel "in the head" with their narrow soundstage. The HE-400se extends slightly past the head, but you won't get the sensation of sound near your shoulders like an HD 800S, K712 or anything like that.
IMAGING
-- Both image very well for the price. I'm able to pick out imaging a smidge better in the 6xx.
TIMBRE
-- This always is a category that's tough for me to assess. Maybe it's my aging, tinnitus-ravaged ears. All I know is that instruments sound real on both. Maybe a bit more natural on the 6xx, but again, timbre is hard for me to judge.
BASS
-- The HE-400se has that "planar bass." I wouldn't call it punchy, but it's more direct and energetic than the bass on the 6xx. The 6xx has present bass, but I wouldn't call it super prominent.
MIDS
-- The absolute strong point of the 6xx. Vocals sound much better and forward on the 6xx than the HE-400se. Any kind of music that's mids-focused will sound better on the 6xx, period.
TREBLE
-- The HE-400se do collapse a bit into grainy treble occasionally with more complex pieces. Still, the treble rolls off nicely. The 6xx, on the other hand, are the easiest cans I've ever listened to because of the treble roll-off. If you get any kind of listening fatigue with the 6xx, then you may have the most sensitive ear canals on Earth. Some people call this roll-off the "Sennheiser veil." Maybe so, but it works very well for my treble-sensitive, tinnitus-ravaged ears. The 6xx are the antithesis of the hot treble of Grados or most Beyerdynamics.
POWER REQUIREMENTS
-- Both need an amp. No question. The 6xx has OK sensitivity but high impedance. The HE-400se has average impedance but low sensitivity. Pick your poison.
COMFORT
-- I have fairly large ears with weird pinnae on my right ear that protrude more than the left. Still, the HE-400se's ear cups fit with ease. The most comfortable ear cups in my collection. But ... the headband created one of the most intense hot spots I've felt for a headphone. Took about five to 10 hours of use and some gentle kneading of the pad with my thumbs to eliminate that. The 6xx have spacious ear cups but not quite as deep as the HE-400se. The 6xx also have pretty strong clamping force. But I still find them comfortable for extended periods of time. The headband is a time-tested, classic comfortable design.
NOTES
-- I have used the HE-400se with balanced cables; I have not with the 6xx. The HE-400se respond well to balanced power, with slightly more detail and better imaging than on single-ended.
-- In general, the HE-400se has a very balanced yet energetic sound signature. The 6xx is a far more laid-back, relaxed listen. I love both equally, and I think there's room for both in any collection since their sound signature are different and one has a planar driver and the other has a dynamic driver.
Let me know if you have any questions. Happy to try to answer!