The old VTL 300s downstairs have seen a fair amount of use and the tubes in there have been percolating for at least a decade, maybe longer. Not constant use but they've been in there for quite a few hours of use.
The degredation was really gradual but I found myself thinking I could use some more powerful amps but maybe I should quit being so cheap and lazy and put in some new tubes first.
I had 14 spare Sovtek 6550s and ordered 3 more, two of the spares wouldn't bias at all (!) and my old DVM went haywire at the same time.
New DVM, new tubes and the power is back, the top end is back, the 3D soundstage is back.
I do have to dial the subwoofers down a bit!