I have an Ampeg BA115T, one year old. At practice last night, the speaker was buzzing -- but only on Db, D and D#, and didn't matter where I played those notes. All other notes were fine. What's up with that???? Is the speaker blown or does it sound like something is loose inside? And how do I check the speaker on this amp? Does the grill pry off or do I have to take the chassis out? Thanks for any help. Got gigs Friday and Saturday and must fix.