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!
Sumhull, As a threshold matter, I assume that the rest of your components are matched well and are in good working order. Just focusing on the tubes, 50 hours by itself is nothing. The real question is how many hours are on the tubes from prior use. I would try contacting the prior owner to learn how many hours he/she put on the tubes.

Alternatively, if a retube is not too expensive, just replace them -- the whole lot. Aside from sound degradation, old power tubes are more susceptible to arcing. By itself, not a big deal unless they take out a bias resister with it. Been there and done that one.

Last comment is I would call the factory to learn what it recommends about tube age and recommended replacement intervals. If you still have a problem, it might relate to electronics which is beyond the scope of your OP.
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...

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.
Elevick makes a fair comment. I agree with the adage "if it ain't broke, don't fix it." Sure, check the cartridge alignment.

However, I do caution that if the power tubes are old, they are susceptible to failure, which in some cases could take out bias resisters. I would still ask the prior owner the age of the tubes. If the hours exceed factory recommendations, I suggest replacement.

BTW, is the amp old?? There's a couple of threads running about cap failure, particularly electrolytics. Just a thought.

See if you get the same results with a different source like a CD player. If the same thing happens with CD's then it may be tubes. Tubes can change with temperature and not always for the better. However, when you own tube equipment it is always a good idea to have spare tubes on hand.
The other thing that comes to mind is your AC power. If everything else checks out ok you may want to try a power conditioner.
Change in sound between parts of a record side have nothing to do with the tubes--that is either something in the recording itself or a problem with cartridge alignment.

Unless you have access to a really good tube tester, the best way to determine when any tube needs replacement is to get a complete set of new replacement tubes as back up. Once in a while, replace the tubes with the new set to see if the sound has changed signficantly. Don't do all the tubes at the same time because that won't tell you which are beginning to go bad. I would start with the power output tubes first (they tend to have the shortest life), then try the small signal tubes. If the sound comes to life with a replacement, you know that the replaced tube(s) have started to go weak. Sometimes, older, weaker tubes also start to become noisy; if a good cleaning of the tube pins does not cure noise, it is probably time to replace the tube.
This is a bit off-topic, but not by much. I have a Sophia Baby that I have upgraded all the capacitors on (I just bought the ones Richard at Sophia recommended). I also bought a pair of Western Electric 396a output tubes. With these few upgrades, your Baby will sound much, much, better, you will be very glad you did them. You can do this yourself, as I did, or send it to Sophia, and they will do it for you. The WE tubes, NOS, are very expensive, so I found a good used pair for much less money. I have been using this amp for about a year and a half, with no problems.

This may or not be of interest to you, but it's something to think about, if you are going to re-tube.

Best of luck to you,
I'd first check the tubes biases whether all are still holding up to minimum factory recommended. Then also your AC voltage, fluctuations/instability of which could lead to the change in sound you described--hence, a stabilizer may be all you need.