I've got a couple year old Streamliner 600 that has been lightly used. Lately its seems that it not nearly as loud. Settings are the same and no change in cabs, cords etc. This could be my imagination I guess. Could I have just gotten used to it and it doesn't seem as loud when it is? No odd noises or anything. Could the preamp tubes wearing out cause this issue. As I said, it's not played out much, mostly in my home. What's an avg life span of 12ax7 tubes? Any thoughts?