I had a vaguely analogous experience with my Sandberg last week. I removed the battery to test it, and it had plenty of charge so I reinstalled it. The next day, I discovered active mode didn't work at all, so I opened the battery box to the smell of overheated electronics, and the battery was extremely hot to the touch. So I removed it, discarded it, let the bass cool down, and installed a new battery. Everything has been fine since then. With the design of Sandberg's battery box I don't think I could have installed the battery incorrectly, and I recalled checking to make sure I had installed it correctly, but who knows. Maybe the battery failed. It had plenty of charge, but was a few years old. I'm just glad it didn't fry anything.
I usually play my Sandberg in passive mode, but apart from a very weak center detent on the bass control, I think the preamp is fine. It's relatively transparent, and I sometimes use it on stage.