"Bracing and dampening a cabinet should follow these basic guidelines.
1. Bracing is designed to divide each panel so as to reduce its tendency to vibrate and move the vibration frequency point upward in the spectrum. A good rule is to divide each panel unequally. In other words, don't run a divider down the middle of panel and keep bracing no more than about 5" apart. That distance really depends on the wall thickness of the cabinet and the panel size."
5" seems like a weird rule. Is that actual bracing that interconnects panel sides (ie front with left right and back)? Or are you referring to hardwood strips (say 3/4"x3/4") securely glued to the panel sides to break up vibration resonances? Putting interconnecting bracing behind speaker cones often creates unwanted impedance "glitches" or aberrations (D'Appolito, 1998). So, I can see 5" working with the strip method but hard to do interconnecting bracing with common midrange or larger driver sizes without creating possible impedance problems...
Glass or mineral fiber board products dampen very well, but I don't recommend them for ported (non sealed) speakers for health reasons.