I've just been listening to mine again tonight. Despite my love for the warm, weighty and relaxed sound of the HD650, I don't think I'm quite done with my K601 after all
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Yes, compared to the HD650 their highs can be a bit too sharp for my ears now and then (I'm pretty sensitive in that area) and I have some albums where the female vocals (and I virtually only listen to female vocals, good old Springsteen is damn near the only exception these days) sound distinctly sibilant with the K601 and not at all with the HD650.
But then I pick a different artist or a different album and the K601 just sound more exciting and much more forward than the HD650 and with none of the aforementioned missteps.
I've been listening to Bat for Lashes' first album tonight, and a few things I were definitely reminded of after using the Sennheisers 90% of the time since I bought them three or four weeks ago, are that the K601 tends to put you right on the stage rather than a few rows back, and that it's not bass shy at all when given the chance, like on "Trophy" from the mentioned album, where it arguably goes just or almost as deep as the HD650, albeit still lacking a bit behind on weight and punch.
I believe the K601 has a reputation for being a good allrounder? Although with my sensitivity to sharp highs and sibilant vocals, I think the HD650 will be the allrounder in my setup (it hasn't failed yet in that regard), and the K601 brought out for music I feel they fit very well.
Regarding burn-in I don't really recall feeling any of my headphones changed much, but then I can also say with certainty that I definitely don't have the sharp hearing that many here at Head-Fi appear to be equipped with. I just tried my HD650 straight out of my iPod for the first time today, and frankly at the volume level the iPod could manage with them - which was admittedly a fair bit lower than my usual listening volume - I was hard pressed to hear much difference from when I've got the headphones plugged into my main system with DAC and amp
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