I have a StudioMix that I picked up years ago on Ebay and used for quite a while until I upgraded to a VS700 (and then to a Mackie MCU after I sold my VS700). I still have the StudioMix sitting around, and I have tried on a few occasions to add it alongside my MCU as a second controller for my synths, but this tends to crash Sonar. It works for a few minutes, but crashes eventually every time I've tried it.
I'm not sure if that's just me not implementing this properly, or if the StudioMix simply won't play nice with another controller, and to be honest, I've never really been THAT motivated to figure it out. So, YMMV, but it didn't work for me.