I recently purchased a new PC (Creation Station from Sweetwater) that uses the third generation Avid Mbox running the latest Window driver (1.0.22). I am running Cubase 6.5.4 on Windows 7 Professional. For some reason, when I try to change the buffer size to control latency under Devices / Device Setup / VST Audio System / Avid Mbox 64-bit ASIO, the control box that comes up has the buffer size button grayed out and says I cannot change the buffer size because an application is streaming audio, which I presume is Cubase. Is there anyway around this? I had this same setup on a 2009 Mac pro and had no problems.
Either switch buffer size before booting Cubase, or try selecting release drivers in background.