For a studio monitor manufacturer to launch a headsphones series is a good sign when it comes to the quality of the product. They are expected to keep their headphones as high end as their monitors and for KRK that's saying alot.
The headphones come in two models: KNS 8400 and KNS 64000.
The main difference is that the 8400s has a slightly wider frequency range of 5Hz-23kHz vs 10Hz-22kHz(Pioneer HDC-2000 however has 5Hz-30kHz).
The 8400 also have a slightly more effective element, a premium Letherette finish vs. a regular and a built-in volum control that the 6400s don't have.
The major difference in my opinion is that the 8400 has memory foam instead of cellular foam isolation. This makes the 8400s have an ambient reduction of 30 dbA versus 24 dbA for the 6400 model. You do want your headphones to be as isolating as possible, cause then you can have a slightly lower volume while performring.
KRK has got the headphones endoced by alot of well known producers. Among them is Rodney "Darkchild" Jerkins who has produced tracks for artists like Michael Jackson, Lady Gaga, Mary J. Blige, Beyoncé, Ashanti, Ciera, Britney Spears... You get the picture.
The KRS-6400 lacks memory foam, but is priced at only $99, so if you just want some headphones for home use. They should be pretty sufficient.
The specs of the KRS-8400 don't match up to Pioneers HDJ-2000, but at a pricepoint of $150 it's a pretty good deal. Pioneer HDJ-2000 is priced at $250. I would recomend checking these puppies out next time you are shopping for headphones. :)