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Firstly, I want to say that the HD381 line is a 'clone' of the Sony MDR-EX90LP; I put the word "clone" in quotes because I actually believe that Superlux is the OEM/ODM for the big companies (Beyerdynamic, AKG, Audio-Technica, Sony, etc.) that it supposedly 'rips off' of.
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For the most part, I agree with earfonia's assessments. The sonic presentation feels great, and the bass extension is surprisingly good. I did encounter some problems, however, and here's where I differ from earfonia's largely positive review:
On certain songs, and it probably depends on the source material, there is this strange vocal distortion that is extremely annoying, like the driver diaphragm has been ripped or something. However, it doesn't happen for most material, so I'm willing to forgive it. The other thing, and this one's not so great, is the vocal sibilance. I bought these as a stand-in for my Klipsch S4s (currently in the repair shop), and one of the biggest complaints about the S4 is the sibilance. At first, I thought the sibilance on the HD381F was less pronounced, but at higher volumes, they turn into these unbearable cracks that just make me want to take the earphones off, even more than I did with my S4s. At least the sibilance in the S4 was more of a Harry Potter parseltongue-type sound, rather than a full on spit-in-your-ear sound. So is this problem forgivable? Well, I'm a forgiving person, so yes. The overall sound is still very nice. Those of you with quick tempers, though, may want to look elsewhere.
At the end of the day, I'm going to be returning to my S4s despite the HD381F's vastly superior midrange and treble. Why? Comfort. The 13.5mm drivers provide the detail and clarity, but to my relatively small ears, they get tiring. I also can't sleep in the HD381F because of its Frankenstein's Monster-type plugs that stick out from the side.