I am in x64 and the Sonar compatability check box is not available in 6.3.1
I think UA just finally ditched the Sonar Compatibility option, as it was a hold over from the UAD-1 days. UAD-2 users never needed to use that option as it only related to specific UAD-1 CPU munching issues. I have been using v6.3.1 since the day it was released without any issues, but then I am on WinXP (SP3) and only use the UAD 32bit versions. Once UA finally releases the 64bit compatible UAD plug-ins is the day I make the move to a 64bit OS.
On the face of it, this appears to be a bitbridge related issue. With UA being on the verge of releasing the 64bit plug-ins, UA may recommend staying on v6.3 or rolling back to it, if you have already updated. IOW, I don't think they are going to waste resources trying to fix a v6.3.1 that will soon be out dated anyway.
Cheers,
Billy Buck