As somebody in a similar boat, I must slightly hijack this thread.
I am a very big metalhead. I listen to everything ranging from rock to deathmetal, mostly metalcore.
I've had the HE-400's, and now have the HE-400i's.
Personally, I found the HE-400's were better for metal. Their sound signature was slightly more fun. The treble was too harsh, which the HE-400i's seem to fix.
Now that's just me, I don't really like the "up-front" sound the mids present on the HE-400i's. I've never heard the HD650's, but many tell me they sound quite similar to those. You may really like that. The soundstage is also much smaller, it makes me personally feel claustrophobic coming from the wide-open HE-400s
I would have kept the HE-400's, but the headband was uncomfortable, and everytime it is remtotely bent, it changed my sense of positional audio. It knocked the L/R sense off balance, which drove me crazy.
Same happened with the DT990, HE-400, and now even the HE-400i. If you bend the top metal headband of the 400is even on accident, it changes the position of the audio, and I cannot bend it back to the center. It's an eternal struggle of mine. I may have to return/sell them, as I find myself not even enjoying music anymore because the off-center positional audio drives me so crazy. I've even contemplated just going to IEMs where this issue won't exist...
Now I must ask, what else should I try. I'm thinking of getting some headphones that cannot possibly be bent out of shape, like the K702, but I am in love with the Planar Magnetic and Open Back Sound. I like the V-shaped sound signature. What headphones do you persoanlly best recommend for metal that you've had or listened to?
I see you've owned a multitude of headphones. What are your impressions of the HD700? Seeing it's sound signature graphs, it seems very similar to the HE-400 and DT990/880. Is it a slightly colder headphone, or did it seem warmer like the HE-400i or HD650? Also how is their sound stage?
My main concern is those puny 40mm drivers... And nobody I know who's purchased them has kept them...
I'm not really ready to give up the space and accuracy that planar magnetic drivers give. I see no other open-back planar magnetics with non-bendable structure on the market though. I am tired of overly fragile constructions. There are no solid-looking constructions better than my HE-400i's on the market expect the Oppo PM series, which is closed back and not as good as Hifiman's.
Not sure where to go from here now, as I'm quite unhappy with the He-400i's sound not staying center and the small sound stage, but I can't go back to the He-400's even worse headband and discomfort :/