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.

I have roughly around 14 sets of headphones & these are in my top 3
i use them daily (along side the dt 770 & hd25) for editing, mixing & daily listening.

-Pros

+Build Quality is super solid
the removable mini-jack-type connection is a freaking lifesaver, it has saved me & musicians from breaking the cable like +10 times

+Fit
the support wings have metal arms so you can bend them to fit abnormally large or small head sizes

+sound quality is superb
its insane for this price, it easily beats other cans 6x its price. i measure (frequency analyse) all my headphones & there are super small differences in the Left Right hi end (+7K) response which adds depth to the stereo spectrum.
overall response has flat mids, 10db sub boost (20-90hz) & 7dB hi end (5K-12K) boost (with a typical 3.5k cut)

+price
you shouldnt be able to break em, but if you do, its not even a big deal

Cons

-Comfort
the original ear cushions are a good quality but they are a to firm for me so i ll be upgrading them with Brainwavez hm5

I really like these phones, but the reason why I bought these was not the reason why I will keep these.

The headphones have a really clear sound, and according to my friend who also uses these. The phones have a straight EQ curve. (with my own EQ settings) The bass is huge. The mids are okay, The treble is okay, but a bit too much for me.

The phones are big, over the ears, huge. They look a bit weird when they are on you, but if you don't care how you look, that really doesn't matter.

These will not break on you, like seriously these are built to last. You don't have to worry about breaking these when you take them off of your head, unlike with some other headphones.

I bought these headphones, so I could have massive amount of quality while on the buss for example. These do deliver in the sound department, but not in the portability. They are a bit heavy and a bit un-comfy. You need to let these morph to your head until they become comfy.

They are really big, so be aware of that! And whilst these are on your head they are borderline UGLY!

I still do recommend these, because if you want big headphones that have good quality for dirt cheap. These are just meant for that.