Superlux HD-681 EVO Black Professional Monitor Headphones

Build Quality: Quite bad, everything you'd expect at this price or worse.
The plastic literally squeaks, the headband is held by a rubber band on the inside, which breaks for a lot of people. The 3.5mm connector on the headphones will break after a couple months if you lay them flat on the table as you normally would, unless you have a stand it will happen so soldering a new connector is necessary.
Sound: Not an expert but these sound phenomenal with some EQ-ing (this makes everything better in my opinion but the bass is much more noticeable).
I love them the most with their pleather pads but many recommend getting velour pads.


4 stars because of the sound.

I just got these yesterday and am impressed enough to write this while drinking my morning coffee, which is quite a compliment. One key point I would make is that while several reviews mention a V-shaped sound signature and strong bass, how noticeable that is will depend on what your previous headphones were. To me, the bass in the JVC S500 I was using felt excessive at times and I was nervous that these would be the same. In reality, the bass in the 681EVO feels perhaps a tiny bit less pronounced and I like it. Overall, I'm impressed with the sound. Vocals sound alive, guitars sound great, drums are okay, and the soundstage feels spacious.
Other bullet points: The accessories are welcome. I switched to the velour pads immediately and prefer them. The 1m cable is great for my mp3 and being able to switch it is welcome. Isolation is noticeably worse than the S500, but that's not a surprise. To my ears, these sound better than my xiaomi piston 3.0.

Worth the money for anyone wanting a cheap pair of headphones. Won't blow anyone away but for $40 you can't do much better.
 
Build quality is lacking and the cables really suck, so these do have a definitive lifespan on them. Mine lasted me about 4 years.