Herman; Thanks for the response. Typically I don't measure a loss of capacitance per se. What I check for physically, visually is actual leakage that is noticible on the capacitors. It is obvious and very easy to see. Others have stated that what they do is simply replace all electrolytic caps. This is also another way to go. I can tell you most electrolytic caps aren't that expensive, but power supply filter caps can really get up there in cost if the rail voltages are high. This is one reason why amplifiers are so expensive. The others are 1) transformers (very high cost), transistors and tube, can get up there in cost if they are speciality devices in low production, 3) machining enclosures, that are well designed and built to eliminate vibrations (i.e. very heavy), rediculously expensive heat sinks, etc. But the only times I typically measure capacitance is when I'm reverse engineering something. or if I simply can't read the lables on the devices because they are so old. But the originally poster's question regarding lifespan of ss amps assumes that there is no routine maintenance of the equipment. Like cars, amps and electronic equipment (that you want to keep operating for decades do need maintenance.) a qualified service tech can do it for you. Replace caps with equal or better quality caps, replace transistors, resistors, tubes, diodes, heat sink compound and thermal insulators, etc. and you basically have a brand new and maybe better amp. But for regular people that don't have the technical background to do this, I recommend that they just enjoy. However, it depends on the equipment. If, for example one gets their hands on the older top-of-the line Audio Research, Threshold, Mark Levinson, Atmasphere, etc. equipment, I would definitely try to service it and get it working correctly and keep it. If going to a "fire sale" and I find two Mark Levinson 20.6 amps, I would do whatever it takes to get those working correctly and keep them. Some older equipment is as good or better then most newer amps today. believe me.
enjoy