How do you know when to replace a tube?


I'm new to tubes. I have a Baby Sophia powering a pair of Klipsch Quartets from an AR ES-1 turntable. The sound is often warm and lovely but sometimes thin and metallic to the point of making me want to stop listening. Sometimes the first couple of songs on an album sound soft and sunny but the last songs sound flat and brassy. The tubes on this amp were the original tubes used by a previous owner; I've used them maybe an additional 50 hours total. Does the sound quality of a tubed amp gradually degrade or do the tubes simply stop working? Will certain types of music reveal a degraded tube or will it be pretty obvious with all kinds of music? Thanks for your responses. I want to get back to all warm and sunny sounds!
sumphull
I'd replace the tubes if you can, have the old ones checked to keep for replacements. I had Quartets for years, and they are some of the best sounding (if not the best) Klipsch ever made. Good luck.
old tubes sound boring and make you want to sleep,

new tubes will sound vibrant and full of life.

That amp should never sound bright, even with old tubes

IMO of course...
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They do gradually degrade but can also just quit working.

That would not be the problem with the sound changing from beginning to the end of the album. That can be caused by improper cartridge alignment.

I'm with the others; buy a new set of tubes and see if the sound improves.
Thank you all for your responses and helpful suggestions. I'll go ahead and replace the tubes. The cartridge alignment idea is really helpful and will give me another project to explore.
I disagree with tube replacement. As said by Philjolet old tubes will get "boring", not bright. New tubes may actually make your system sound even worse, depending on what you buy.

2 things I can think of:
First-Klipsch can be fatiguing after a while. They may sound great at first but after 15-20 minutes your ears grow tired. These are really accurate and sometimes overly-revealing speakers.
Secondly-It could be your cartridge alignment. If the first half of every album sounds great but once the need approaches the halfway point or so on an album it may be tracking funny. THIS IS MY GUESS. Make sure your cartridge is properly set up by a pro.