I also own the 668b and the 681evo. I would recommend the 681evo. The 668b has a little more upper mids than the 681evo, but I don't think anything is missing in the 681evo mids, no loss of detail and it sounds a bit more natural to boot. The bass is more powerfull in the 681evo, but that also has something to do with extension, the 681evo goes noticeably deeper into the low frequencies than the 668b. The highs are somewhat lower in the 681evo, but that is a good thing because there is too much treble in the 668b. The 681evo has more neutral and more natural highs along with the fine balance in the mids.
Build quality is better with the 681evo, much better pads and a better headband.
There is no doubt in my mind, you should clearly go for the 681evo. (Not that the 668b is a bad headphone, the evo is just better in every way, that's all)
Thank you for your comparizon. I've already rejected HD668B thanks to few nice reviews of Superlux family. Indeed I'm going to go most probably for the EVO but I am also weighing up HD681B now, because one guy here on the forum (@Natsukawa), who owns all Superluxes, said that 681B have slightly bigger sounstage and better instrument separation than EVO while smoother trebles unlike HD681. So I am a little bit confused which one to buy since I like EVO's design more and they sound phenomenally good for their price but still I can imagine that older one often overlooked 681B might be slightly better in terms of sound. Seems like I won't be able to choose one untill I test them in a market (when I have time for that).
Edited by Jeger - 1/12/15 at 9:09am