Lifespan of a quality solid state amplifier?

Bifwynne, 6 months is nothing. You can safely plug it in. Caps go bad after at least few years unpowered.

Spinaker01 - Drying of the electrolyte increases ESR of capacitor. When it gets into many ohm range it becomes very audible and after that it might even go to thermal runaway since current going thru cap x ESR = Heat but temperature increases ESR (positive feedback). Smaller caps are connected in parallel making ESR lower to start with - less audible when old.

Each 10degC of temperature increase cuts life of capacitor in half.

Electrolyte inside of a cap eats out aluminum oxide (dielectric) lowering breakdown voltage. Presence of voltage rebuilds this layer.

Bifwynne, No input signal is OK. Tube amps need load, SS amps don't.

SS amp might fail when input signal is present and output shorted (overcurrent). I've heard that some poorly designed class D amps might go into oscillation without load but likely will survive that.

Tube amp might fail when input is present and output is open (overvoltage). It might even happen without input signal if amp goes into oscillation.

I increase volume slowly, at first, making sure I hear the sound from both channels. It is good habit, IMHO, since things tend to get shorted or disconnected between listening sessions.