I have a triple X head, and I just didn't play it for a while because I was busy with work and stuff. But I've been playing it lately, and the distortion is just not cutting it anymore. There is no definition in the low end, and the treble sounds fizzy and scratchy. I'm thinking it's the tubes because I bought it used and it was three years old when I bought it. But I'm not sure because the power and distortion is there, but it just sounds horrible.
I'm far from a tube expert but I would definitely say that you do need to replace them.