There are many different Koss headphones, so it depends on which particular one you're using. And, to be honest with you, I believe the quality of Koss took a "dump", in recent years. I had a pair of Koss from nearly 10 yrs. ago, that gave perfect clear crisp sounds (where you could hear flutiness, and so forth, and extremem range of tones, etc....). Eventually, they gave out after nearly 10 yrs. So since I had enjoyed them so much, I went out to Ebay and found an un-opened pair of those exact same model # headphones. Imagine my surprise when the sounds were muffled, and not as good, as my previous pair!
Before sending them in for "repair", I sleuthed around and got the following explanation: You can not expect that Koss (or any other such manufacturer) gets their parts from the same vendor source, year after year. Because in their quest to protect the "bottom line", they might .... over the course of time ....... choose a less expensive vendor, for the components inside their headphones. And even though the spec's might/should be the same, they'er not always. So for example, maybe for a few years early on, they got their interior speakers from a US manufacturer. But then later on, switch to a vendor from china, to save money on those "same" speakers.
I ditched my Koss headphones and went with a pair of Killer B's (which are like Grey Ghost). If you have any friends who have Killer B's or Grey Ghost, see if they'll let you plug them in to your machine, and do some side-by-side air test comparisons to your phones.
840 VLF/TR. Then I have owned a 
5900, 6000Di, 6000Di Pro, XLT and a DFX, 
CZ6 CZ6a, CZ7.
in 2000 with a XS . 
Then I have used and owned the XS-S, SE.