Pros - Compared to RUK30: Slightly better vocals, A little more refined bass & mids, Bright sounding treble
Cons - Treble attack duration is a little short, Price difference is too much between RUK30, Falls behind of KDX100 in overall sound quality
Pros and Cons section pretty much explains everything. i advice you to read my updated ruk30 review first to get an idea about this earphone: http://www.head-fi.org/products/koss-ruk-30k-noise-isolating-in-ear-stereophone-black/reviews/11943
there is not much to talk specifically about kdx200 here. it shares nearly the same driver with ruk30 and costs 3 times more while offers less improvement than it's price difference suggests. however, it's not a complete step back or exactly the same device at least (unlike the brainwavz s1 vs s5 situation). but things are getting more complicated when we add kdx100 into the equation. i'll make a review of it soon.
koss advertises it's product kdx200 with the words "accurate acoustic reproduction" on koss.com. there was absolutely no information considering the sound in ruk30's description, so at least this is something... there's only a few reviews about this product. one of them belongs to head-fier james444. here's the parts i found accurate in his review:
"drums and bass are strong, but well controlled and without the common boominess"
"Mids remain clear and seemingly unaffected by bass"
"Soundstaging is also very good, with realistic dimensions and forward projection"
"Vocals sound natural and slightly forward, without getting too close"
it seems james444 was very impressed from the treble performance of kdx200, maybe because he didn't used a better performer on the price range, e.g. ostry kc06a. i don't say kdx200 treble is bad but it has it's up and downsides. it's still a bass & mid performer after all and despite it's bright and refined treble, it just cannot represent a fully detailed, long enough high frequency attack. actually this situation also continues with kdx100 so it's a usual koss thing but kdx100 is still better overall. so yes, it has a quality, bright treble but it just not enough to compensate it's announced price and kdx100 doing better for lower price. this is my final conclusion about kdx200's treble.
on the other hand, most complaints from james444 were about the bass presence which actually could be solved with equalizer settings and i really don't understand such statements... how they're able to get so much bass from a flat earphone ? are they using a bass boosted sound source but they don't even aware ? this is a total mystery to me. i did searched about james444 on the forum and i reached to the information that he wasn't using any eq settings previously but later he decided to use equalization. whatever...
james444's review is also shows this device as a $50 earphone and his review is dated 2011. koss.com selling this product for $80 right now but it can be found as low as $65-70 on other sellers. interestingly, koss products were much cheaper in turkey for some time. i bought this device for $60 but later it's price fall down to $35-45 on some sellers. i don't know exactly what's going on and why they decided to do this after i bought it but with this kind of pricing, i've to say that koss is really crushing other companies. it's very sad that people are not even aware of it.