300Bs - does leaving them on shorten their life?

If they are on, they are being used up. 

Clear and to the point! Thank you!!

There is so much debate about this because powering on is probably the hardest thing you can do to tubes. Personally I always turn mine off when I am finished listening for the day but leave on as long as it is possible I WILL listen. So approx 4-8 hrs on days when I listen to music on my system (not every day).  I would think that if you spend significantly less time listening than not listening turning off is a good way to go. My amp is also class a so it is always full power all the time. So that is a factor for me. 

No it's not a good idea. You need to be there or at least in listening range.. It's just the nature of the beast. Those are exposed bulbs, Valves, or tubes.. Be wise.

They don't always fail and keep it in the glass, get my meaning..

I've seen an valve fire or two. LOL TRUST ME the first one is an eye opener.. I was 15.. I learned QUICK.. Almost burned down my mothers house. I had to pay for the carpet I destroyed.. That wasn't funny. Mac MC20s and a MAC 30 Kit.. BLEW a 6L6.. caught the carpet on fire..

I was trying to power a Jensen Imperial Sub.. It worked great.. UNTIL...
My mother was a saint...

regards