I just got my pair of ATH-CKN50's to compare to the CKM500's. I've had my CKM500's for several months now so they're fully broken-in, but the sound between the two is very similar. The only major thing I can discern at this point, until the CKN50's have some hours on them, is that they have deeper, tighter bass than the CKM500's -- though slightly less of it. The highs on the CKN50's are crisper (though not harsh or sibilant), but I recall my CKM500's being that way at first too. The big thing for me is that the CKN50's have smaller housings so they're more comfortable to wear. They have the exact same nozzles as the CKM500's so the same sleeves (Complys, etc) will work with either. The only thing I don't like so much about the CKN50's is that they have an asymmetrical cable; after the split, the cable going to the left earpiece is shorter than the one going to the right. (This works OK for me since my Audioengine N22 amp is to my left on my desk, so the cable needs to head that direction anyway.) The cable material is exactly the same between the two earphones, but it's just one long cable for the CKN50's vs. the short-cable-with-detachable-extension like the CKM500's. Also, the CKN50's don't include the nice black storage bag, though they do come with the same assortment of replacement sleeves.
Overall, I'd say you can't go wrong with either earphone. I've listened to a decent number of earphones in different price ranges (Shure e2, SE210, SE425, inexpensive Sony in-ears, etc.) and these AT's are the only headphones of any sort that I've been perfectly happy to listen to without needing to use EQ. Set your source to flat and enjoy. Right now I'd have to give the edge to the CKM500's for sound quality (just barely), but the CKN50's for comfort. It's just reassuring that what I'm hearing from these two appears to be AT's "house sound" so the CKM500's aren't an anomaly in their earphone lineup. If they keep it up, I'll be buying more AT's in the future.
Also of note is that Audio-Technica US has just picked up the CKM500i for distribution. They have an MSRP of $99 but should go for less in stores. I remember paying about $70 for my CKM500's (non-smartphone version) when I imported them from Japan, so I'm going to guess we could see the CKM500i go for about $80 here.
Thanks for the comparison DrWeb!!! Only thing that would annoy me is the asymmetrical cable, I really dislike them!!! I'm blessed that the 500's cam with a normal cable!!
Really interesting to hear that the sound these ckn50's produce are close to the 500's, I'll be interested to hear about the future versions of the ckn series like the 70's??!