Ok. So I have a 10 year old Mike Lull. Apprently the preamp went out. I've tried it with 3 different batteries, all tongue tested first. The output seemed low the few moments before it went out. Now I can't get anything from it. This was at soundcheck. I had the volume on the bass all the way up as I always do. Luckily I can run it in passive without a battery so I can still use it but would like to fix or replace the preamp. What's the most common causes of this?