What can one say about these headphones. I really couldn't believe that you could find such sound, comfort, especially in closed back, at such a price. To explain, I come from hd555, hd650, lcd2 classic, m50x, fidelio x2hr, and iems fiio fh3, fh1s.
COMFORT: The most comfortable thing I have ever worn on my head, simply put, you can wear them for hours, super lightweight. Comfort ranking k371>hd555>hd650=m50x>lcd2 classic>fidelio x2hr
BUILD: Decent. lcd2 classic>hd650>fidelio x2hr>k371>m50x>hd555
BASS: Tremendous bass, specifically sub-bass that easily reaches 20Hz and is slightly more boosted than the Harman target. The mid-bass doesn't bleed into the mids. Of course, it doesn't reach the texture of the lcd2 classic, but it definitely satisfies. lcd2 classic>k371>fidelio x2hr=m50x (bleed into the mids)>hd650 (no sub-bass)>hd555 (Bass? What is that?)
MIDS: Good mids, they are a bit grainy on some tracks. hd650>k371>lcd2 classic>hd555>fidelio x2hr>m50x
TREBLE: Perfect for those who are treble sensitive, sacrificing a bit of airiness. They are slightly recessed and not fatiguing. lcd2 classic>k371>hd650>fidelio x2hr=hd555>m50x
SOUNDSTAGE: Personally, I found them quite wide for closed-backs without having a great depth. lcd2 classic>fidelio x2hr>k371>hd650>hd555>m50x
IMAGING: They lack a bit here, they are a bit hazy. lcd2 classic>fidelio x2hr=hd650>k371>m50x=hd555
VALUE: Extreme. k371>hd650>lcd2 classic>fidelio x2hr>m50x
DRIVABILITY: You won't need an external amplifier, they perform satisfactorily even from your mobile phone. They have low impedance and high sensitivity. k371=m50x>fidelio x2hr>lcd2 classic>hd650
They were driven by Fiio e10k and Fiio k5 pro amplifiers and the Fiio m6 DAP.
TL DR If you are looking for headphones in the sub-300€ category, look no further. If you would like a slightly milder sub-bass and high treble, I would try the same company's k361 at an even lower price.
PS for the price of the hd650, I would easily go for the Hifiman Sundara.