Aluminium electrolytic capacitors do degrade with time. Many electrolytics have a vent for allowing excess gasses to escape. This escape can result in the electrolyte drying out and the performance of the capacitor falling.
Also if aluminium electrolytic capacitors are left for a few years, then the oxide layer on the anode can dissipate.
source: https://www.electronics-notes.com/articles/electronic_components/capacit...
It's often a good idea to replace electrolytic capacitors that have been sitting around, not having voltage applied for a long time. At least that is the "conventional wisdom". I suppose it is more important to do so if the capacitors had been used, and then not used and also subjected to wide temperature variations. This describes my amplifier, between sitting in storage in New Hampshire through the winter and summer (below freezing (-10 C), to the 90's (+30 C)) as well as during one spring in Quartzsite (100+ degrees).
I looked at them and they look like new. I found them, the exact part number, on Digi-key and decided to order them and have them on hand in case there is a problem when I power it up. About $80 for the set of four. You can see these, the blue capacitors under the power supply board. They have screw terminals, which you can see on the top side of the board near the pink resistors.
Speaking of "powering up", I ordered a Variac to use with the power up exercise. $100 new, from Amazon. The plan is to bring it up at low voltage - maybe 20-50 VAC to the input and look for smoke and sparks.
...continues at https://www.99hobbies.com/buying-the-variac
