I'm dumbfounded by people here saying they don't like these. I have pretty high standards: the WORST headphones I use on a daily basis are HE500s, and that's only when I'm tired of my Alpha Primes and my Stax Lambda Pros. I don't think very highly of dynamic drivers in general, but the KDE250 here is one of only two dynamic driver sets I actually like (the other is a Sennheiser MX985).
These 250s legitimately compare with the TOTL Audeze iSINE20. They don't sound as good as the 20--does anything this size?--but they come impressively close. I actually sold off my 20s because the 250s are close enough in terms of overall SQ, and are actually BETTER for portable use. The iSines are really unforgiving of bad sources, they will show you all the flaws in a recording, but they scale up with better equipment beautifully. The KDEs, OTOH, sound great even from a smartphone's headphone jack. The Koss won't scale nearly as high as the Audeze can, but they do still improve pretty significantly on my desktop rig.
For reference, my desktop rig is a Schiit Mimby/Magni stack. When I'm not at home I sometimes listen just out of the headphone jack on my V20, other times I'll carry a Hifiman HM601 or an ODAC/Fiio e11 stack with me (which actually rivals my desktop stacj in SQ). I listen to EDM, hip-hop, and occasionally rock.
Honestly I question whether people like jamtato below me have actually heard a KDE250 before. I can certainly understand disliking them based on fit or comfort or aesthetics, or maybe they aren't a good match for your specific use-cases. The 250s can be sensitive about where exactly they fit on your ears, because there is a definite "sweet spot" where they sound their best, and they sometimes require frequent adjustment to keep them there. I understand all that, and those are valid concerns. They aren't appropriate at all for runners, for example.
But people are complaining about the SOUND??? It's one thing if they simply don't match the sound signature you prefer, which I suspect may be happening, where idiots confuse subjective opinions with objective performance, or contrive the latter to make themselves feel better about feeling the former. I think the HD650 sounds like complete shit, personally, but I don't go around telling people that it sucks or that it sounds tinny, because it fucking doesn't. And neither does the KDE250.
I can't help but notice that NOT A SINGLE ONE OF THEM actually recommended an alternative that they consider better.