I have a Gallien-Krueger MB 150III, which is a great combo amp for rehearsals and small acoustic gigs. I use a 1970 Fender Jazz bass with a custom make ebony fretless finger board and it sounds great with the MB150. I bought this combo with the idea that I could add my old gray Ampeg B-15 bottom with a JBL K-140 speaker in it and would have a great, larger sounding rig for bigger gigs, punch form the 12" GK and the old, fat sound of the B-15. Well, it sounds awful with both cabinets. If I turn off the GK internal speaker the B-15 sounds good. With both on, it sounds like the cabinets are cancelling out each other. Has anyone had this problem and have any of you used an extension cabinet with the MB150 with success? I asked GK and of course they said that I should buy their MB 150 extension cabinet, which is made precisely for this. But, I don't feel like laying out $300 just to "try" something. Any thoughts? Thanks in advance.