My 20-39 cs plus causes the grout in my bathroom to crack, no joke! I was in disbelief when the first cracks appeared and i dismissed it as a bad grout installation. Even after posting my problem (and also some pics) in another forum to ask whether a sub can really damage grout, the general consensus there was no it can't. Well the grout guy came buy and fixed all the cracks. I was a bit weary about turning up my sub from then on. But one day i popped in gladiator and blasted it. After that session low and behold there was this crack on the grout that was never there before! My mom got pretty pissed about that and i agreed not to blast my sub too loud anymore.