It interesting that you say the xb90s have more rumble, as that's the one thing i thought the CKS77 lacked. Bass is too clean on the cks77, lacking texture, which makes the xb90s even more intriguing.
Yes i agree the cks99 by all indications seems to be an improvement over the cks77. The slightly larger driver should deliver more bass and dual chamber historically employed on the cks90 is fascinating. I hope the soundstage will be even bigger.
Just had a look and dont think I can buy them yet in the UK.