JBL C38s and amplifiers.

[Hula]

Dynaco ST70 with a modern driver board, for the win.

What pre amp do you use?

 

[Hula]

How long do tubes last and how do I know when I have a bad tube?

 

[Nyjblbob]

All amps and preamps use the tubes differently, So there is no real answer as to how long tubes last, How often you listen, Most power tubes will weeken before the Preamp , tubes.
But again that is Amp Specific. How does a tired output tube sound?
that could be a thread in itself. But unless a single tube goes out, most will agree change all 4 tubes together.
A new Quad of Output tubes make a huge difference.