Taking appart the price and forgetting the price to loose the Hype i must to say that his quality sound it's good, at the same level of cans with my HD555. Differents sound signature but same quality level .
The very best sound of this cans cames when we burn in about 200-300 hours, recesed mid came more forward and don't miss it soo much, and the initial bright blows up a bit but don't dissapears. At his 56ohm and sensivity is not a requisite amplifier the HD668b to make it sound good but an amplifier make it push his best sound, resolution, instrumental separation, etc.
In other words they are more Mini Giant Killer with a good amplifier on the way.
It's supposed to be balanced cans for the B on HD668B but it seems to me it's not balanced flat, it's balanced on "V" like the Beyerdynamic DT990 -on the superlux website it's frequency response it's compared and it's very similar-, soo take in mind that, this cans aren't Flat at all.
This cans are very analitycal, very detailed with envolvent sounds and with a very good soundstage, all this make rediscover new things on music you use to listen; little sounds that wasn't before but appear with them, it's a good sensation rediscover your old music on it.
They are very transparet on the recording quality sound, if you find distorsion don't blame on cans, blame on bad recordings and mastering studios or the bad music quality music you heard. Most pepople says this cans are bright and i think it's beacause of it, if the recording it's bright the HD668b make you to know "screaming" to you
The bass its well extended and present, maybe a bit muted on the very low frequencys (20-50HZ) but much more present on middle low frequencys (50-150hz) and with a very funny punch on it. Many people will found it short of bass but i think its the perfect quantity to feel it real and not overbass.
I found the punchy bass very very funy and enjoyable on it. But without amplification dont expect miracles...
The mids are a bit recesed at first but after 200-300 hours it came a bit forward and you don't miss it anymore, unless you like very forward mids in that case you don't like it. They are very well balanced after the burn in, very detailed, with clarity and natural.
The highs are very very good for me -but i like crispy sounds!!-; some people found highs very very present on this cans and don't like it for this reason, it's a point to have very present on it. I think that the bright highs dissapear a bit after the 200-300 hours of burn in but always be there.
The "timbre" it's not bad at all, is not the most realistic -my Sennhesier HD555 have a more crystalline "timbre"- but i like it soo much on cord instruments and female voices and acoustic concerts. Maybe not as realistic as my Senn but much more fun and enjoyable.