Superlux HD-669

Bought them for only monitoring live recording levels at church.
First thing i did was to compare them to my ATH-M50x. Here are my thoughts:

Build quality:
Nothing no praise unfortunately. Manufacturing marks, cheap looking thin cables, cheap-feel plastic, ugly carrying pouch and "leather" headband pads.

Comfort:
These are huge! Pads feel stiff and earcups can only move few degrees to adapt over your ears. But this can be a good thing when you happen to have slightly bigger ears.

About the sound:
I found them very bass heavy, but bass is quite tight and defined. But for my taste, too much bass.
Low-midrange is quite "missing" and what's left is muddy.
Upper-midrange is good! Vocals sound very clean, nice and present through these!
There is a very audible presence boost say, about 5-8kHz range. It gets a little annoying and tiring when listened on high volume for some time.
Unfortunately above these frequencies there is mud again. Highest frequency details, often referred as "air" or "shine", are poorly defined and sound dull.
They are not very sound proof either. Outside bleeds in and otherwise. Wouldn't recommend for tracking.
One more thing, compared to ATH-m50x, they sound "distant". While ATH feels really close, in your face, HD-669 creates "distance" between the the ears and the sound. Can't say if it a good or a bad thing.

Power hungry!
They do need a lots of power to drive them. When plugged straight to smartphone, you cant get anywhere near loud without distortion. But my MacBook Pro preamp did just fine.

FOR THE PRICE I would say amazing sound! Useable for a lots of things, but for serious mixing or everyday music listening, please consider something else. In blind test I would have guessed them in 50-60 euro price range by the sound.