How do you know when tubes are done?

That's about right for the power amp tubes to be showing their age. I would go to TubeStore.com and select a set. Read the descriptions, they seem quite accurate, but bear in mind there is just not much difference even from the very best to the very worst. There is however a pretty big difference from old worn out to brand new!

Don't try and do one or two, replace them as a set. Do the power tubes first as that is almost certainly what's going on and even in the unlikely event its not after 8 years you'll want to be ready with a set anyway. While you're at it, or later, you could do the pre amp tubes. They're cheaper and last a lot longer, but again better to have a set and not need them than to need them and not have em.

Oh, how do you know? No two are quite the same. One can suddenly burn out in a blaze of glory. Or die when you're not looking. If they don't do that, old worn tubes tend to lose dynamics and slam and sparkle. But it can be so slow and gradual its hard to notice. Or fairly sudden. Sometimes they will start making a kind of static noise like fabric pulled across a microphone. fffffttt.  fffffft.  Then nothing. By that I don't mean nothing it dies I mean nothing the noise stops. Its intermittent. Which can happen with any tube at any time even brand new. If it starts happening more with tubes that weren't like that before that would get me thinking maybe they're getting a little long in the tooth.

That's why I like to keep a spare set on hand. Any time I get to wondering in goes the new set. If they sound about the same that means the old ones are fine, and the new ones go back in the box for later.

I agree that the best approach is to keep a new, or tested NOS, set of tubes to put into the amp for comparison purposes.  At some point, the tubes have to be changed, so it makes sense to buy the set now.  You could get "lucky" if the set you buy now goes way up in price by the time a new set is needed.

By the way, tubes are never "done."  They can always be sold on ebay "as is;" if they you have a set of totally dead tubes, you can sell them as a "matched pair."