Do amps degrade over time?

"01-16-13: Lowrider57
Also, since your amp is 15 years old, it was built in USA."

Indeed. On the back it says "Made in Snohomish Washington".

Entropy happens!

As you state capacitors partially electrolytic do degrade over time, a hum is normally the first sign that a cap is on the way out. Visually it may show signs of leakage or swelling.
I would look first at the power supply section, it’s likely a main power cap is at fault.

Other components can also degrade a little, like resistors, more so Power types, but it’s unlikely you’ll find many of those in a modern amp these day’s. Most solid state devices tend to last, unless shorted out or they over heating for any reason, if any blow it’s often due to a another faulty component down the line.
It seems your amp has two large 20,000uf electrolytic capacitors for the power supply, so I would say they need changing. It’s well worth having it serviced. Good luck.

Yes the sound degrades. Get all the capacitors replaced and it will sound as good as new maybe even better after new, better replacement capacitors are fully broken in.

Transistors also degrade over time but it usually takes longer than 15 years. By the time the unit is 30 years old one can expect a higher noise floor, not only from the transistors but also the resistors. Corrosion can also cause semiconductors to fail for no apparent reason. This is usually a problem associated more with ICs but it can certainly affect discrete devices also.

For now though, since you already have the power supply filter caps replaced, I would expect no worries for quite some time.