setting up Cubase 7 with Mackie Onyx 400f

Quote:
“when I maximize cubase, the distortion comes back.”

Could that be a graphics card issue?
What does DPCLat.exe tell you?

Have you tried disabling windows sound? Maybe Mackie’s rudimentary driver can’t deal with two clients.
Which reminds me- have you tried unchecking the “release ASIO in the background” box, in CB’s devices menu?