I own an Ashdown Evo II 500 bass head that has recently had a sudden huge drop in volume. I'm talking so quiet that you can easily talk over it, even when it's absolutely cranked. It doesn't sound distorted or really bad in any way, just an incredibly quiet clean sound. I've ruled out anything wrong with the bass, cab or cables, and made sure the mute button wasn't pressed. Any ideas what could cause this? A blown fuse? A busted pre-amp tube? Anything else I might not have thought of?