Hi, I'm keenly interested in the outcome of this as well, since I'm also looking at the exact same SIIG card, plus looking to purchase a Saffire Pro 14.
I found a thread at gearslutz, the guy seems to be having a similar issue on a Pro 40, same SIIG card, Win 7-64. But it's an older thread (MixControl 2.4/3.0). Ultimately it turned out to be a bad mobo... HOWEVER, he mentioned something interesting about when testing it out with the legacy driver, that he had to manually delete individual saffire drivers in the system32 folder because a simple uninstall doesn't delete them.
So while MixControl 3.2 may not need legacy drivers, a broader issue might be with any uninstall procedure itself, where the uninstall routine leaves traces of the previous drivers that have to be manually deleted.
The other thread:
I hope you get this sorted out, keep us updated.
Edit:
Ugh I'm too new a member to post links.
The thread can be found if you google the term "saffire mixcontrol nn-e20012-s2 goat gearslutz"