I have an Acoustic 20 mini combo which I really like, and that I've been using since August. It has a peculiar problem. When I hit the E or F notes on the G string the cabinet in the area of the 10 inch speaker produces a buzz and a kind of out of phase vibrato. It doesn't matter if I use my Casady bass with round wounds on it, or my Hofner with Pyramid flats. But, it goes away if I turn the cabinet UPSIDE DOWN. Nor does it make these sounds if I lay the cabinet on its side. Clearly, I've tried various things to isolate the cabinet and to de-couple it from the floor. I never play this loudly because I play with an acoustic guitar finger picker and a few singers. I have no idea as how to get the grill off, and I hesitate to do so because the amp has a 3 year warranty. Any practical suggestions, or speculations? Thank you.