Replacing tubes on used equipment bought on A'gon

Investing in a tube tester may be a good idea if you have a lot of tube gear. If you have a local club they too may have a communal tester that you can use.

Depends on type of tube. Tubes degrade gradually with use, but will "work" considerably longer than their ideal useage time. So you could have a tube working 5-10 years that musically degraded after 2-3 years. The only way to know is to try new tubes and see if you hear a difference. If not, you have a back up set of tubes.

I would find out what the manufacturer requires for stock tubes and purchase a set for each. This way you'll have a good idea of the voiceing intended. From there you can descide for yourself whether its worth the tube rolling or not...Have fun !