How do you know when tubes are done?


I have a homemade pre-amp and amp - both tubed.Recently, it became necessary to turn the volume  up quite a bit to get the same sound level.It still sounds good but I started to wonder if a tube or two was the culprit. The tubes are about 8 or 9 years old and  get light to medium use.
Suggestions?

steamboy
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Usually a browning of the getter flash indicates a weak tube, but there are a few types in which this is not the case and the tubes can still be viable. A black thick getter flash is fine. I think it was GE 6L6GCs that had a browning tendency that turned out to not indicate problems.
You sneak. Is the Yee a one-off custom or regular product? Either way kinda cool.
I have a Croft pre and power amp. I had the ffffff noise from one speaker, dropped the amps off for Glenn to service, the tube had reached the end of their lives, had them replaced. Sounds amazing now. However there was no loss of volume Glenn described it as a noisy valve! 
Thanx Veridian! My first ST-70 cost $79 in kit form. Can't remember what the built price was. Those M-125 amps look very interesting maybe for a small system up stairs. Let's see...Rogers LS3-5As. TD 124 with an SME on it. A PAS 3X. Just like old times:)