Did'nt really see this in the Amps FAQ ( the subject, printed in blue, means it is being edited ), so I wonder if we could get some views. I recently mentioned in the Portaflex thread that I was going to replace the tubes in my '67 B15N, and got some discussion going about why I was even considering it, to the difference in some brands of tubes for the same model. But what I'm really wondering is, how long do tubes in a bass amp usually last, and how do you know when to change them? Should they be changed as a set, or individually as needed? I was told that some guys have tubes in their amps that are 40 years old, and don't see the need to get new ones, or NOS tubes. My guitar player ( Fender Super Reverb ), changes them every 5 years or so. So, what do you think about the life of tubes, when should they be switched out for new, or NOS models, or, are they good until they finally die? What about new as opposed to used, or NOS tubes? Anything else you want to add, feel free.