Hi Eric!
What I am finding is that the CKS1000 is a kind of wonderful hybrid of the WS99 and the AD900X. The bass has the same ws99 characteristics in that it is big, bold, extended, layered, detailed yet beautifully controlled. There is a lovely amount of sub-bass, and it is never boomy and never bleeds into the mids. Completely satisfying bass, just like with the WS99's, and something I had never experienced in an iem before. The mids are somewhere between the ws99's and the ad900x, but I find that the soundstage, instrument separation, imaging and layering is amazingly more like the ad900x's in that the instruments are all over the place - left, right, close, far away - amazing for an iem! So I guess the mids are more like the ad900x's. And the treble is energetic, engaging and detailed like with the ad900x. So that combination quite frankly makes for a rather epic sound... end game iem material for me quite frankly!
But, you are probably more interested in the WS99's since you don't have them, so here you go.. The way the 99's handle the bass is
beyond good (just like with the 1k's!). Friggin' gorgeous bass. I can't describe it any better than that. For modern, electronic or bass oriented music, they are mind blowing and can't be beat imo. For everything else, they are only awesome
The mids are neither recessed or forward - just right. The treble is very nice, but not too bright (which I personally like). I would overall say that the ws99's lean toward the warm side, the soundstage is great but only as wide and deep as closed cans can be. But they are very nicely balanced. In summary they have a gorgeous, full, balanced, immersive, hypnotic and easy to listen to signature. I can't imagine closed cans sounding much better to be honest.
If anyone wants to feel completely satisfied and stop the bleeding and not feel the need to continue getting more and more gear, then just get the open ad900x's, the closed ws99's, and the csk1000 iem's. ATH holy trifecta.