Here are a couple things to try.
I once took my amp apart, to fix a buzz, and discovered that the problem was my input cable.
The next thing to try, you have to be extremely careful not to damage your hearing. Put your ear close to the amp and try to figure out exactly where the buzz is coming from. This might help you chase an air leak. The MB150 has a couple of large screw holes in the sides. Do those holes have screws in them, or are they open? You can cover holes with tape.
When you removed the speaker, did it have a good gasket?
Finally, it could be something in your room that's buzzing. There's a cabinet in my living room that I have to stay away from because it buzzes.