SWR LA15 I picked it up last week and have been pleased with it except for the fact that even at half volume, you get a pretty intense vibration sound when you play the G and A notes on the E string. And open E causes no such problem. The volume isn't even high, just about 1/2 way and the bass boost will be in the middle (no boost) and it'll still do it. Could it be a sign of something wrong? or just the fact that it is only a sub $300 amp.