Ah, shame I didn't see this thread earlier as I was in the same situation as you a few months ago when I owned the CK100PRO as an upgrade candidate for my aging CK10. I could've told you that the spike in the highs at around 8kHz (you can find FR graphs here http://en.goldenears.net/15507 and here http://sonove.angry.jp/sudio_technica_ath_ck100pro.html) is not to be dismissed and that the sheer amount of treble energy (ie the shelf from about 2~10kHz) from them was an issue for me, and I would assume would be for most others as well. Thus, you end up with something that can definitely be classified as a fatiguing signature whereas the CK10 as you said is much more balanced and easy to listen to. I've not heard the CK100 before but I'd venture to say that the CK100PRO type of colouration is even more objectionable than whatever direction the CK100 went in terms of its signature. The upgrade from CK10 to CK100PRO is somewhat reminiscent of moving up the Grado lineup where the general consensus is that the SR325 and RS1 are much brighter than the rest. The reason I say this is that the CK100PRO sound signature actually reminded me of the SR325 at first listen. The lean sub bass and the treble emphasis is something both share. I believe it's because of this top-heavy, uninviting signature that prevents the CK100PRO from being a more popular top-tier IEM choice.
However, I would have also added that in terms of tone and timbre, something that FR graphs fail to communicate, the CK100PRO clearly bests the CK10. Across the entire frequency range, though to a lesser degree in the bass frequencies simply because of a lack of sub bass presence in both, music sounds more organic and natural coming out the the CK100PRO than the CK10, which sounds flat and lifeless in comparison. This was the most pleasant surprise for me auditioning the CK100PRO, because I've always thought that discussing tone and timbre is kind of moot when it comes to BA IEMs since traditionally these characteristics are handled poorly by them. But that said, I've since come to equate impressions of tightness/leaness to an inorganic bass, at least for BA earphones, and I would say this is still the case with the CK100PRO. In the end, they're still no match against dynamic IEMs in that department, and that is why I've moved away from BA IEMs in favour of dynamics. On that note, though I'm wary of comparing BA to dynamic IEMs, the Sony MDR-EX1000 is nothing like the CK100PRO. It has ample sub bass and the treble is much smoother. There's this weird spike in the high-mids at around 5kHz that makes cymbals sound kind of unnatural though. Anyways, I shall refrain from writing too much as the CK100PRO have been traded away for a while now (still holding on to the CK10 for now) and what I write here are the very apparent differences in performance between the two that are quite hard to forget.
Edited by yaluen - 8/27/13 at 8:16pm