If you are looking at older amplifier, whether tube or solid state, after about 15 years or so you will want to consider replacing the filter capacitors in order to maintain the reliability and performance of the amplifier.
Often filter caps will go longer than that, but after about 15 years their reliability is suspect. Certainly they will not be meeting the specs that they were originally built to.
If you don't replace the caps not only will the performance of the amplifier be reduced but you run the risk of damaging the power transformer. In some cases this can be a very difficult part to find and it is one of the most expensive parts in any amplifier, so this is something to keep your eye on.
Often filter caps will go longer than that, but after about 15 years their reliability is suspect. Certainly they will not be meeting the specs that they were originally built to.
If you don't replace the caps not only will the performance of the amplifier be reduced but you run the risk of damaging the power transformer. In some cases this can be a very difficult part to find and it is one of the most expensive parts in any amplifier, so this is something to keep your eye on.