well, your post is couple of months old... I wonder whether have you managed to test both the HD380 and the KNS8400 since.
Because I did today. They've just arrived with the post.
I had about half an hour to listen to them.
I really do hope that these phones will get better with some days of burn in - if not, I'll have to send them back. In the past 3-4 years I've been only using my 30 years old AKG K240DFs. I just thought it's time to get some new HPs, too. These two the HD380 and the KNS8400 are miles away from the DF.
The HD380 out of the box sounds really cheap. From the upper mids up everything just sounds crippled. Everything is detailed up there though ... just very very silent. Bass is boomy... like a subwoofer in an accousticaly non-treated room.
The KNS are a bit more promising. They are really really comfortable. Very crisp highs. Bass is OK ... clearm but really light (for me that's not a problem, I have to check the Bass on monitors anyway when mixing).
At this stage I'd say the HD380 is a cheap Hi-Fi headphone (I'd say thy are in the same league as the HD555, though the HD555s are much more comfortable - sitting on the head and for the ears) . the KNS8400 is definitely made for the studio.
I am really really curious how they'll sound after couple of days of burn-in.
I was just thinking about value for money... if these cost around 100 Euros - the K240DF (imagining that they are still in production) should cost at least 400 EUR.