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!