I have written a review in danish at the danish head-fi-forum, which is called Hoved-fi.dk, and I'll now do my best to translate it into english:
I have on a few occasions heard of Superlux, and that their headphones, despite being low cost, aren't the worst one can buy.
A few days ago, I had the opportunity to buy a pair of HD 668 B's for what equates to roughly 21.50$, including the cost of shipping and a pair of velour earpads.
Naturally, before recieving the headphones, I had seen quite a few written and video reviews of the HD 668 B, but since I do not seem to share sound preferences with a whole lot of reviewers, I am never quite sure what to expect from a headphone.
Finally: The arrival of the HD 668 B's! I removed the stock "leather" pads without even trying them with the headphones, as all of the reviewers mentioned that the stock pads were awful. The velour pads are very comfortable, and because the headphones weigh next to nothing, they are also comfortable for longer listening sessions.
The build quality regarding the rest of the headphone can best be described with the words: "You usually get what you pay for"
Except for the headband and the holders for the "wings" that rest on your head, the headphones are made of plastic. They dont squeak or rattle when handled, but apart from that, there aren't many more nice things to say about the build quality.
Not that they are going to fall apart with everyday use, but on the "nice-materials-scale", I'd give them a 2 out of 10 - because there at least is some metal included in the construction.
The headphones are sold with (I believe) a 10 foot and a 3 foot cable. I got mine secondhand, and only got the longer of the two cables.
The cable is rather thin, but is at least detachable, so if you happen to step over the cable, it should just come right off - if not, the cable can be replaced as described.
The 668 B's make use of what I'd describe as "wings", that rest off the top of your head, and prevents the headphones from sliding down. The concept is fine, but you might have to bend the wings (carefully!) a bit, in order to adjust the clamping pressure on your head.
The sound:
I have owned a pair of Hifiman He-350, that I'd - before owning the 668 B's - describe as being bright. HD668 B is, however, even brighter, and a bit more aggressive.
If you'd like a bit more lowend, you can try adding more layers of foam in front of the drivers.
If you one day find yourself with 30$ you don't know what to do with, I'd recommend you try out the 668 B's - If my description of the sound seems like something you'd like.
I am not going to go more in-depth about the sound, as I do not consider myself to be experienced enough to properly describe other sound characteristics - and I do not want people to be mislead by improper descriptions
