Wayne Parham Messages: 18792 Registered: January 2001
Illuminati (33rd Degree)
You're right, only two sets of braces are required, one 1/3rd the way from the top (just under the midwoofer) and the other 1/3rd the way up from the bottom. They should have insulation sitting on them, spanning the cross-section.
That photo was one of Shane's builds, and he put in an extra brace. That's OK, but not required. The reason I posted this photo was to show how the insulation siting on top of the braces spans the cross-section of the cabinet.