Hi guys, I'm new around here but I just got my third Line 6 amp, a Spider Valve MkII HD100 tube head, about two months ago. I love the amp, but I got it from a friend who had owned it for two years before me without ever replacing any of the tubes. I already know that I should replace my tubes as it is, but today at band practice I experienced my first ever problem with it. I turned the amp on, let the tubes warm up like I always do, and when I flipped the second switch, the volume was incredibly quiet. Even when I turned it up farther than it should ever go (both the master and channel volume), I was still able to talk over it. Occasionally, the volume would spike up to being loud for a second and then go right back to being quiet, but after about 20 minutes, the volume spiked back up to how loud it should have been in the first place and stayed that way. Every so often it would drop back to being quiet but then it would spike back up to being the right amount of loud again.

So I guess the bottom line is: do I just need to replace the old tubes, or do I have a more serious problem on my hands?

Thanks!