The responses related the electrolytic capacitors re-capping is accurate. However, it isn't just the power supply caps. All electrolytic caps should be replaced when recapping. In my opinion.
Power supply cycling add stress to capacitors. This is mainly true for equipment that do not have standby circuitry or slow start circuits. For standby circuitry, the caps are pretty much charged all the time or in a lower state and as with slow start circuits, current and voltage is slowly applied (relatively speaking). Therefore, there is much less stress on the capacitors. So, to me this is a non issue in modern electronics.
Heat is another issue for failure of electronics. properly designed and constructed electronics with proper air flow/cooling helps prevent heat related failures dramatically. Again, a non-issue if this is employed.
I would be more concerned with Class A solid state amps, where the bias current is such that the amp's output transistors are fully "turned on". in other words biased such that the transistors are operating at or near full output current with no input signal. this means that components are getting a lot of heat, which significantly lessen component life.
Another good thing to do with solid state amps that one wants to keep is to replace or upgrade the pre-drivers and output transistors with exact replacements or upgraded transistors. Not only are the newer transistors more linear than the older units, they (in my opinion) sound much better.
I've upgraded many older amplifiers this way. The transistors must fall within the same general specifications as the older one. such as voltage ratings, current ratings, hfe, etc.
Also, equally important is to at least remove and replace the old transistor insulators with new insulators and silicon thermo insulating compound. I've found that the compound dries out over time and the insulators dry and crack and cause shorts between the transistors and the heat sinks. This, to me is more important than re-capping.
I wouldn't worry about the replacing resistors unless I see an obvious problem.