As quoted from Polk's website on their Lsi's.
Side panels are 1-1/2" thick for totally inert enclosure - you hear all the music and only the music.
I see the Lsi9's and 7's have full enclosure of side panels but why not the the 15's,25's, and center ?
I'm sure the drivers on these towers/center will resonate some sound coloration ? So why not have those enclosed with the side panels?
Asthetically, the look is probably better w/o this addition but just a bit of curioisity that's all.