Superlux is an interesting company. They're from Taiwan, though most (if not all) of their stuff is made in China. Some of their headphones are home-made designs while others are very clearly inspired from others. The 669 look exactly like some Sennheisers and even the sound is not too far off, though the price is close to half or 1/3rd of a comparable model.
Takstar is kinda similar, their Pro-80s are basically a Beyerdynamic clone for 1/3rd of the price.
On head-fi there are plenty of review about their headphones, in recent years Superlux has become a favorite for value-oriented gearheads. Even on Youtube or Reddit it's possible to find plenty of positive reviews of their products. A cool thing about their headphones is also the cable:
Look at it. LOOK AT IT. Absolute perfection. If my Denon's had that kind of detachable cable they would still be alive. Feelsbadman.
I cannot say much about their open cans because when I wear headphones is usually for sound isolation (both for me and people around me), and when I'm by myself or watching a movie with dem wifez I have my 5.1 at home so open headphones would be a bit reduntant.
Build quality of Superlux's gear is...well, ok for the price. They probably spend most of their design budget on the drivers and the rest is mainly cheap plastic and cheap pads. When the pads of the 669 will start to peel off in summer (because I'm 100% sure they will) I'll probably replace them with some velour pads for better comfort.
The 669 were the biggest headphones I tried from Superlux, some of the smaller models (especially the portable ones) felt terrible on my ears but that varies from person to person.
For anyone interested in trying them on: http://www.taiwanaccess.com.tw/
This is the official retailer in Taipei. Their prices are a bit higher than what's available on Ruten, but I was glad to pay the extra bit in order to try the whole range of products.