It's the capacitors. Caps last around 20 years. If you replace them BEFORE they die/break/explode, you'll have another 20 years.
Agree 100%.
Electrolytic Capacitors are a fact of life in amplifiers, tube or solid state. They last about 20-25 years, which is a sort of half-life, IOW about half of them will be bad in about 20-25 years.
I would add that if you buy an amp that has not been in use for several years then it can increase the risk that a cap will blow up when you first turn it on. If you store expensive gear then I'd suggest plugging it in once every six months to a year.