I have tightened down all the screws on the back panels, placed rubber washers between the speaker and the amp and still get this annoying buzz on frequencies ranging form F sharp to about B, on the low E string. Could this be a speaker problem? I did just install what I thought was a good Celestion Greenback in this amp ... which almost touches one of the EL84s, so I am wondering if that is it. I guess I will have to remove the back grille to test that unless someone has other ideas. Thanks.