Wow!
Just plugged these into my Audiotrak Im-Amp and they sound great straight out of the box. Based on the sounds I'm hearing now I'm selling my HD650's and recouping/releasing the funds for other interests.
To my ears, they really are that good.
I have a BIG head and found the clamping force of the HD650's rather tight but the HD-668B are comfortable from new. I might get some plushy pads for an AKG or something to reduce the sweatiness the faux leather pads will no doubt induce.
SOUND: Straight out of the box they have a more complete sound than the HD650's. Maybe its the lack of the Sennheiser 'black' veil? I think its probably a slight lift in the mid-high frequency range. But thats fine by me as the so called 'telephone band' is where we as humans are most sensitive. Bass has been said to be light but I put this down for the fuller mid-highs.
Insight is incredible, given the price and obvious effect it has on build quality. On the first 2 or 3 songs i played I noticed things I'd not heard before. Notes played on 'real' instruments begin and end just as you would hear a real instrument to do. Vocals never seem to push into sibilance which I found an issue on some recordings with the HD650's
As noted above, these are good enough for me to have sold my HD650's
BUILD: Lots of plastic, but are 'high end' phones really any different? Not really.
My only real bugbear is that the cable options are rather limited as the HD-688B have a 3.5mm jack on the left hand cup and you have to connect a cable from there. Sure its easy enough to make one up but I'd like to be able to wire straight up to the driver. Never mind - for £35 inc shipping I cant really complain so I'm off to listen to them again.
GIANT KILLER? Maybe, maybe not. But good enough to leave the £300+ phones alone for a good long time.
Lets call them 'THE BEST ALTERNATIVE'
Just wait 'til they break in (around 150 hrs) - you will be addicted.
Many of us 668B owners mod them...PM me if you get the urge...
Steve