I have the 6i6. And I also just upgraded from X3 Studio to Professional so you and me are rowing the same boat
When you installed the new version did Sonar ask to import your settings from your old x3 version? For me nothing at all changed, I'm still working on projects from X3 and all looks and works the same. As a matter of fact, I have accidentally opened x3 about a dozen times now and started working and to tell the truth I don't even notice!
I think I read another thread by you where you have re done your computer so possibly your starting from scratch?
I have the Mix control shortcut in my lower task bar so it's always easy to open.
It should open from the Sonar Audio preference button too. That button turns into the Wave Profiler if your not in ASIO mode so if the button is there then all should work.
Make sure you install the latest Mix Control Firmware. It is now on Version 1.8.
It sound like what your saying is you have sound, but the mix control is not showing any action on the far right output mon 1/2 meters??
If all else does not work I would un install and re install the Mix Control 1.8 driver.
http://us.focusrite.com/downloads?product=Scarlett+6i6 Hey cool, I see they released a Windows Xp driver now. Not that I need it but at least it shows they even care about the die hards of computing. I can use it with my 2004 Laptop for live recordings instead of the Tascam.