Have you donated to Eugene to help his development costs?... AFAIK the KX Mixer is currently not allowed by the NT 6.2 kernel, he's probably working on it but it'll be slow progress as he likely doesn't have an RTM of Windows 8 to develop against.
Well, it worked on consumer preview... hardwareheaven.com/general-discussion/214980-windows-8-consumer-preview-64bit.html
Works fine for me. I tried installing 3550 on a fresh install of 64-bit Windows 8, not an upgrade. The installation works like it should, without errors. After the installation, it wants to reboot the computer, and after the reboot, there will be an error dialog shown after logging in and going to the desktop, but that can be fixed. At that point, dismiss the error message, right-click at the bottom-right of the screen, and start the Task Manager. Go to the "Start-up" tab. You will see an entry for "kX mixer". That's where the error came from. Right-click on "kX mixer" and choose Properties. Go to the compatibility tab, and set it to Windows 7. It will now start without error message, and everything seems to work like you remember from that point on. I set it up just like in Windows 7, default output in the Control Panel is changed to SPDIF/AC3 for my soundcard, etc.
Kx works in win8 with that little fix, yeah. ASIO doesn't though. It doesn't install properly or something.
There's a note about that for the 3551 alpha version, saying it supposedly fixed something about ASIO not registering on 64-bit Windows. So if you had that error with 3550, you should try 3551.