The ATH CKW1000ANV went down to a acceptable price range these days.
 
If good sealed, the sound is all right over all. Something like "no frequency or behavior of this thing is really disturbing"  (but over all clearly behind Sony 1k and Fidue A83 for sure)
Best attribute: You can hear long time without getting tired. Not this bad, though.
 
Conclusion:
If it fits, give it a try (if you can get it for a fair low price.)

In short words:
 
The Fidue is on par with the Sony EX1000 (little less sound stage and depth, but better, more flat sound signature)
 
 
Downside:
Cable plugs to the earphone speakers was worn out after very short time.
They has play and lost contact then. Maybe they was not resistible against (my) sweat. ^^
 
I switched to Ultrasone cables with identical mmcx connectors, then. They work flawless for the moment, but it doesn't look and feel like the pretty original cable. (Ultrasone cable goes 360° round about when plugged and has a simple plastic look)
 
Contacted my dealer in CN and Micheal from Fidue. But no service, help or spare parts till now.
Not acceptable for inears in this price range. Sorry. This is pretty dissapointing, otherwise it'll be a 5 star rating
 
Update: Michael from Fidue was a little late with his answer for some reasons. But he did, and it was not only a "better late than never" thing. He also contacted his UK sales man.
They shipped me "spare parts" for free, much more than I expected! I'm a happy customer again
 
This is what we are talking about. A premium product and a premium support. Now 5 stars are the only rating I can do.
I won't use them for extreme sports anymore, but for everything else they are top notch next to my Sony 1k.
 
 
 
.
 

Tested the XBA4 vs. the EX1000. So, this might be not the most fair comparison, but shows where (all?) multiway drivers lacks.
 
No doubt, there are highs, mids and deep tones, the only thing that's hard fo find is music.
Especially if you like voices and wide / open soundstaging. Compared to the non BA, one way dynamic predecessor top model EX1000, I instantly get hitted by the extreme differs in overall sound quality and musicality.
 
Even my old Klipsch 2 way BA Custom 3 is much better overall. (also it has a little less base and highs, but it makes emotions / music)
 
IMHO Sonys new top model XBA4 is a extreme draw back. O.K. you have to pay only the half price than the EX1000, but it's not worth it.
To be fair: It's not only a Sony problem, there are many multiway BAs out there with the same shady construction, they all got phase problems caused by to many crossoverpoints and drivers. Some manufacturers do a better job building 4 driver BAs, but none of them made a 'close to perfect' one / reference type, cause it's simply impossible. 
 
Hope this trend will end soon (like multiway loudspeaker boxes in the 80s), let's see when it ends ... 6 drivers are 'state of the art' now, when will be the point of return? 8 drivers 4 ways? 10 drivers 5 ways?
 
O.K., enough slagged. Built qualtiy is O.K., but they don't feel comfortable on the long run, sound is adequate for tech / electronic music. You'll find better choices in this price range, better look for other constructions, less ways or dynamic drivers.
 
 

To make it short:
Get one if you find them, the EX1000 are Sonys best inears, maybe for a longer time.
Use a fitting player that can handle the impedance, and you get breathtaking results.
One of the very best soundstages I ever heard, no BA comes close to it.
 
Frequency response ist good, highs can be sometimes 'special', but it is not a big deal. IMHO it's more problematic if the player is sensible to the slight impedance peak around 5 kHz. Try it out. Also try different eartips ... but you know, for sure.
 
Some people say, the EX1000 is close to $ 1k customized BAs, and so I do!
It's a real bargain now, and a shame, that Sony discontinued it.

No doubt, the 668 is a great value.
The sound is on par with some 200$ cans, and therefor really impressive.
 
But it is far from reference headphone level, specially the mids and balance are not even close. Built quality is surprisingly good for the costs, but still flimsy and made out of rather non elegant materials. The cable plugs are a nice gimmick.
 
Is it a winner? Definitely!
Is it close to Beyer 880 or Senn 600 class? No, the 668 sounds really good for 50 Ohm type, but it is far from the constant quality and the comfort of the upper class.

 
Knowing the 600 and 250 Ohm Beyers (880s), both are pretty good headphones and one of the best values in the market.

But the T1 got a better soundstaging than the 880s, is as fine in the highs, or even more detailed than the 600 Ohm 880, and solid sounding in the base like the 250 Ohm 880, and much more resistant.

Driven with a solid amp in balanced mode, the headphone is a absolute beauty.

No headphone is perfect, so the T1. He needs a little equalization to get even better. (Like Senn HD800 and others can getting better with a good EQ / digitally EQ)
Notching the 8 kHz peak a bit (-5 dB) and between 80 and 400 Hz a little (-3 dB), and there it is: My heady for the next few years.