Although almost all the advice in this thread has been good, I think we're making the mounting technique too convoluted. If you have 6" of stud depth, just run wires to the speaker location and install the sheetrock. Then cut the proper holes for the speakers and mount them in the drywall. It's not brain science or rocket surgery. If the studs are too close together, you have to mount the subs elsewhere. The speaker locations in your drawing were pretty close to ideal. And don't drill into the speaker enclosures. MDF won't accept screw threads like wood and you risk ruining the cabinet. There is no real need for new construction brackets, although I used them in my office to guide the learning-impaired drywaller. Use the integrated installation brackets and install the speakers into the drywall. Also, if you don't use the NCB, you have the option of moving the installation location after the room has been drywalled.
aka TRIAD DUDE
I can tell people who are judgmental just by looking at them...