300Bs - does leaving them on shorten their life?

After you break them in, say 60 hours (no content needed for break-in, just leave them on), IMO it is STUPIDO to leave them on. Other than that I have no opinion on the matter.

Turn em on, go do something else for a few minutes, play, OFF.

Some equipment has built in protection.

My Cayin  tube Amp has a 30 second delay, test and I suppose you could call it a soft start.

My prior McIntosh SS amp has a test/delay protection sequence.

My Adcom ACE-515 power enhancer (not a power conditioner) power controller (in my office) has a time sequenced 10 sec turn on delay for the amp; 30 sec turn off delay for accessories. (prevents thump to speakers).