life span/failure rate of filter capacitors?

Each 10degC temp increase shortens capacitor life by 2.  It is simply drying out of electrolyte.  Drier capacitors have higher ESR.  At certain point current thru capacitors causes internal temperature increase.  Temperature increases ESR and higher ESR increases power dissipated inside increasing temperature further.  It is Avalanche effect that can cause explosion.  To prevent serious damage all electrolytic caps have some form of pressure relief - either rubber plug or cross cut that weakens one side.  Hot capacitors should be replaced as soon as possible.

@audioman58 

caps get dried out as they age .

and does that statement apply to caps in units that have sat unused for a number of years?  My amp didn't see much (if any) use from '08 through '10. and in '10 I officially unplugged it and it was dormant until nearly the end of 2017.

@yoyoyaya 

It kind of depends on whether you believe in preventative maintenance or are a kind of fix it when it breaks person.

I generally like to fix stuff before it breaks.  Particularly stuff that might cause a lot of damage when it breaks.

@jasonbourne52 

Electrolytic cap lifespan is unpredictable! My Sumo Andromeda is still going strong while my BEL 1001 and GAS Son need new caps.

how much life is on the caps in those units you mentioned?

+1 kikanki 
If you want the amp to last, replace the capacitors. Have someone use higher temperature rated caps whenever possible. I have amplifiers that use components that are unobtainable so preventative maintenance is a requirement for me.