Pros: Decent, neutral mid-range. Bright treble with great clarity and detail. Very bassy.
Cons: "Cold" presentation. Treble too sharp (sibilance-prone). Bass is boomy and loose, not very well-defined or tight.
My pair is several months old and fully burnt-in. Bought them for my wife after reading all the rave reviews on Headfonia and Head-Fi.
Presentation and Build Quality
I've read some complaints about the plastic being "cheap", but IMO the nice matte finish hides away a lot of that. Quite frankly, I don't think the HD668b deserves any criticism in terms of construction quality or materials, being priced at $50 - it is miles ahead of any other audio product in its price range in these aspects.
It's very obvious that Superlux took elements from both AKG and Audio-Technica designs (I wouldn't be surprised if they did OEM works for other headphone companies before creating their own product lines, like a lot of Taiwanese companies). The AKG cup designs are simply awesome, and I've been a long-time fan of AT's "wing support" design for the headband, so both are a big plus for me. Add those on top of the two sets of detachable cord, a 1/4 in adapter, a carrying pouch, and an anti-yank clip... I was impressed.
The cardboard box it came packaged in is fairly plain, but I like it. Overall a very professional presentation.
I have absolutely no complaints about the comfort of these headphones. I have a fairly large head and I personally don't feel the headphones clamp too tightly (a lot of people seem to have an issue with this). The earpads are very large and goes over the ears without pressing on them. My ears do not hurt from prolonged usage of the HD668b.
On the subject of earpads, the type of pleather used on these cans are much higher quality than most other "cheap" cans I've seen. They are very thick and durable - a bit on the stiff side, but not uncomfortably so. They also don't stick to the skin and make it sweat like the cheaper pleather used by most budget cans.
Very satisfied in the comfort department.
This is where I have a bit of mixed feelings about the HD668b... perhaps because I had too high of an expectation from all the rave reviews. To be fair, the HD668b still beats the crap out of any "cheap" headphones I've heard, but since everyone calls it a "giant killer", I was expecting to be absolutely blown away, and I wasn't.
Treble: Right out of the packaging, these headphones were extremely sharp and unforgiving. This problem improved a bit after about full burn-in, but the HD668b remained very sibilance-prone. Granted, there are a LOT of details - but the peak around 7-10k is an issue for me. It does have a lively, sparkly presentation, which I do like - I just wish it were less sharp and sibilant. The mids are also too recessed to complement the sparkly treble properly.
Mid-Range: The mid-range is too recessed for my liking - especially since I am used to the warm sound signature of Audio-Technica. That said, the quality of mid-range is good - it is very detailed and not too thin. I just wished it were warmer and more pronounced.
Bass: Before I bought these, I've read several reviews saying the HD668b isn't bass-heavy. My experience is the opposite - it produces too much bass. This may be where my biggest disappointment with the HD668b lies. It's not so much that I dislike bass - but I like fast, tight bass. The bass on HD668b is loose, boomy, lacks definition and punch. There seems to be a peak in the mid/upper-bass that has the tendency to overpower the mids, which is already recessed.
Sound Stage: The sound stage for the HD668b is quite spacious, and this I do like. Sound is also very focused, so positioning is good.
Instrument Separation: The HD668b performs well here; the drivers seem to have good resolution so all the details in the treble and mids are present. Orchestral music is properly layered and instruments are easy to pick out.
Isolation: Isolation on the HD668b is passable - not any better than the cheap $30 full-sized cans my wife had before this, but not bad considering its semi-open nature. It should be fine for private use in homes or for sound work at a quiet studio. Do be aware that HD668b has moderate sound leakage, however - and may annoy people around you in libraries and whatnot.
When I cleaned these headphones for my wife, I noticed that the fabric sitting over the drivers is actually padded by fairly thick pieces of foam. Since higher frequencies tend to be easily affected when passing through solid material, I looked up HD668b modding on Head-Fi and realized that people have been removing them and replacing them with thinner pieces of fabric.
My personal solution was cutting out two large "rings" using a piece of cardboard, then stretch a piece of pantyhose over the cardboard rings and then stabling them together along the edges. Then I used a pair of scissors to trim off the extra fabric along the edges. The cardboard rings sit very snugly under the earpads and are completely invisible, and they keep the fabric nicely stretched to allow sound to pass through unobstructed. :)
The resulting difference is minor but satisfactory - the treble sounds a bit more balanced, and the mids seemed to come out a bit more. Bass didn't improve, however - but I didn't expect it too.
The Superlux HD668 are a very good pair of headphones when seen in the perspective of its price range, superb design, solid construction, and great comfort. Its audio quality is not perfect - in fact if you let the hype surrounding it get to you, and expect to be blown away, you may end up being a bit disappointed. But again, for $50, I think criticizing the HD668b for its audio quality is just being nit-picky. It is great for what it is - a cheap, good-sounding, well-designed and constructed pair of headphones for newcomers in the audiophile world to experience music properly before they eventually move onto more expensive options down the road.