I have a Hartke VX215 cabinet that has a low frequency resonant buzz that is very loud at lower volume settings. It occurrs when I play a low A, B, C, and C# only. At first I thought it might be a speaker problem, but after removing the grill the problem went away, so it appears that it is a problem with the natural frequency of the grill. Has anyone else had this problem? All the grill screws/spacers are tight. Right now I have taken off the grill since I am just playing at home, but it just a matter of time before something damages the speakers. Otherwise I like the sound of the cabinet which suits my modded Epi EB-1 very well. I have a GK 800RB powering it, by the way.