I have a Fatman iTube mk2 Carbon and it's been working very well for a few years. Now though, the right speaker is very quiet.
I know it's not the speaker itself as I swapped them over. So the next thought that occurs to me is whether it's a problem with the valves.
When valves need replacing, do they just completely break (and therefore give you no sound at all) or can they deteriorate over time?