I have the B20, It has been a great practice amp for about 2 years now. I also I have a problem with it. The power switch quit working a long time ago so I plug it in when I need it and unplug it when I don't.
You have to remember these are not the best of amps, that is why they are so cheap!!! To me it sounds like a loose wire(much like my power switch) I wouldn't worry about it, the amp still works right and probably will continue to do so, it just a light, don't sweat it
besides if it was losing power you WOULD know it.