I have an ampeg B2 combo with a 15". When I play the combo alone it sounds fine. But when I run it through an ampeg 4x10 cab at 350 watts (as opposed to 200 out of the combo alone), the 15 makes a buzzing noise when played at a high volume. I tried tightening the speaker and it seemed to help for a short period, but now I here the buzzing again. Any suggestions? Also, the 4x10 cab has a knob on the back of it that has -16dB on one side and a 0 on the other, it can't be for volume because it doesn't become louder when I rotate it, it just sound more trebly as I approach 0. Does anyone know exactly what this knob does? Sorry about the lenghty questions, if anybody gets this far.