Summary: I'm going to level with you -- I bought these headphones for a reason that probably nobody in history has ever purchase the KRK KNS 6400s. I bought them because I thought I might be able to coax a good bass response out of them.
Pros: Excellent detailing, very well rendered mids, surprisingly good sub-bass, responds well to EQ, comfortable headphones, high isolation
Cons: Even after EQ, lack of bass impact due to an overdamped bass response, very unmusical with stock settings
Summary: KRK KNS6400 A detailed monitor
So I had to give the KNS6400's a shot because they were recommended to me based on my love for the SRH940 and other headphones that are all about mids and details.
Summary: Most of the magic from the KNS 8400 is retained in the KNS 6400 design. You get a rock solid set of headphones with a pleasing neutral sound signature. Some of the design changes hamper the performance somewhat, but even so they are still remarkably good value being priced at a mere $100.
Pros: Excellent price / performance ratio, Closed back design, Decent amount of noise attenuation, Comfortable to wear even over an extended period, User replaceable cable, Solid design, Good gaming performance, Excellent bundle
Cons: Not quite as efficient noise dampening as on the KNS 8400s, Cable is a bit too long for portable use