Pros: Excellent clarity in treble and mid-range. Decent bass quantity.
Cons: Sharp, bitey, very sibilant highs. Mid-range is recessed (V-shaped presentation). Bass a tad loose, and extension is poor.
I bought these after reading all the rave reviews on Headfonia and Head-Fi... here are my impressions of them.
Materials & Craftsmanship
I think any criticism concerning materials or craftsmanship of the HD668b is nitpick considering its price of $50 (easily found for less) - I personally haven't seen headphones in this price range that are this well built.
It's obvious that Superlux took elements from both AKG and Audio Technica designs. The way the earcups pivot is very nicely implemented and conforms easily to the shape of the head. As for the Audio Technica-styled "wings", I've been a long-time fan of it and prefer them to traditional headband design, so that's a plus for me. Add these on top of the two sets of detachable cords, a 1/4 in adapter, a carrying pouch, and an anti-yank clip... I am honestly quite impressed considering the 668b's price.
The packaging is simple but elegant; a very clean and professional presentation. The cords are carefully wrapped to prevent movement during shipping.
Construction is very sturdy. No rough seams in the plastic and nothing squeaks or rattles. I personally find the comfort level to be excellent as well; the wings provide just the right amount of down force and clamping force is just right. The pleather used on the earpads are above-average quality and are fairly soft without the stickiness of the cheaper pleather types.
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. Since so many calls it a "giant killer", I was expecting to be absolutely blown away, and I wasn't.
Treble: These headphones were extremely bright and harsh-sounding. It's strange that the review on Headfonia claimed the treble was excellent - granted, there's a lot of detail and when it behaves, the treble is quite sparkly. But as you can see in the FR graph there is a very sharp and pronounced peak between 5-6KHz, which is THE area that causes sibilance (overall the entire treble region is just way too pronounced to sound natural). I am not sure why Superlux tuned this headphone to accentuate this particular area, but I agree with Tyll's (from InnerFidelity) assessment on this one... it's like listening to a good headphone through razor blades.
Mid-Range: The quality of the mid-range is good, though a bit recessed. I think in spite of the issues with treble, the 668b's drivers are quite resolving and the mid-range does carry good clarity. It's just unfortunate that it tends to be drawn out by the distractingly sharp highs.
Bass: Contrary to the Headfonia review, I don't feel the bass is particular tight or controlled on the 668b, and bass extension leaves a lot to be desired (as you can see in the FR graph, the bass starts rolling off at 60Hz, and I certainly don't feel much sub-bass from these cans).
Sound Stage: The sound stage for the HD668b is quite spacious, likely owing to its detailed treble, clear mid-range, and its semi-open nature.
Instrument Separation: The HD668b performs well here; the drivers' good resolution allows the details in the treble and mids to be heard clearly. 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.
The Superlux HD668 are a good pair of headphones when seen in the context of its price range. Design and construction are superb, as is comfort. Its audio quality is a mixed bag, and if this really is the only entry-level headphone you can afford, then be advised you will need significant EQ to make it sound pleasant. Then again, for $50, I think the HD668b is still a good deal as long as you don't mind having to EQ them. Personally though, I would save up a bit and invest in something in the ~$100 price range for a proper introduction into the audiophile world. First-gen Sennheiser Momentum, AKG K553, Audio Technica ATH-M50, or a used pair of NAD Viso HP50 are all excellent headphones that can be had for less than $150.