Life span of internal SS amp parts

It depends on the design and hours it was played.
12-13 years is a long time for a power amp which runs fairly hot, even if the design is conservative.

You can definitely expect to have both of those amps serviced just like if you bought an older Mark Levinson amp. Heat kills components, Krell uses proprietary output devices but stocks replacements. Rowland fixes their older equipement also and has parts. Just watch out for manufactures that aren't in business anymore. Older Class A biased Thresholds are door stops because the output devices no longer are made and don't exist in common channels..
Also just because a component wasn't used doesn't mean that it will last longer.. I left a Carver amp in my closet for 1.5 years and the caps dried up in the powersupply and it cost more than the amp was worth to fix.

Great Question!

I agree with the others. The hotter it runs, the shorter the life span. Capacitors are generally the first things to show signs of wear but they can still work suboptimally, whereas transistors just die when they reach the end of their life. But as Cytocycle points out, not using a component for extended periods is bad too.

Arthur