Amp Upgrade - Capacitors, Resistors or both?

Generally speaking, capacitors should be replaced after a few decades.
Resistors, I can't say.
If you give your location, maybe some members can recommend an amp specialist near you.
B

Yes it is more or less true. Electrolytic capacitors in particular deteriorate and have a safe life expectancy of perhaps 10-15 years (more recent ones from mainstream manufacturers are better). Do not take chances with those in your power supply because if they fail you can blow up your speakers and/or have them leak onto your system board. So have at least the power supply caps replaced - this is easy. Modern electrolitics tend to be more compact, so you may be able to fit larger capacities in the same amplifier case for a bit more dynamic headroom (this can be audible, particularly with smallish amplifiers). Resistors also deteriorate but that is pretty harmless. A good design allows for wide tolerances in components, so sonically there is not too much to worry about (unless you are an audiophile hypochondriac).
I am using various older Quad amplifiers and these have all been refurbished professionally by a former Quad NL service engineer. He had a simple policy of always replacing all resistors and capacitors for a complete refurbishment: the components are mostly dirt cheap and time to test them is money wasted. Avoid boutique parts.