I just got the HTF600 this morning. Here's my impression after a short time of listening. In short, I'm impressed.
I listened to some songs on Pod by The Breeders. Razor's Edge by AD/DC, Big Ones by Aerosmith, and a couple of other genres.
I like the tonal balance, they are the darkest sounding headphones I have, The treble is subtle, they're quite bass heavy, kickdrums sounds awesome. They sound smooth and not grainy. There's a good body to the sound, and instruments, such as guitars, have good weight to them. I like that a lot. As far as negatives goes, they sound a bit closed in/congested, but that's hardly surprising as they're a closed can (or is it semi closed). I think they're very enjoyable to listen to, fit is good, and they're very lightweight
tdockweiler wrote earlier:
"Sound would randomly go from crystal clear to almost muddy depending on the song."
- Yes, I get that feeling too. On the other hand, I think that's the case of most, if not all, headphones to some degree.
I just did a little shuffling between the Superlux HD681 (with practically no bass in the right driver due to recent damage), Sennheiser HD650, HE-6, and the HTF600. After the HE-6, which I think is in a league of its own, I think I like the HTF600 the best, at least this morning and because they're new.
The Superlux have a brighter tonal balance with more treble in comparison, and the Superlux is probably more detailed and accurate with better instrument separation. But they're are beaten by the HTF600 to my taste. At least with the songs I've listened to so far.
The HD650 has a bigger and more open sound, and a better sense of layers in the sound. The tonal balance is brighter too. I'd say the HD650 is undoubtedly the better headphone, but that's doesn't mean the HTF600 is bad in comparison, it's just different.
The HTF600 will be replacing the Superlux as my portable headphone, so it's a good thing it looks better too. I'll be using them quite a bit at home too I think. Overall, the HTF600 is a fresh breath of air to my collection, as I've been wanting a treble-light warm sounding can for a while.