Questions from a Tube Preamp newbie

JUST A NOTE ABOUT CRYOSET...Ron Sheldon is very ill and has sold off his tubes and other gear.  A terrible shame.  I have had great performance from his cryo'd tubes.  Unfortunately bad things do happen to good people.😔

I left my VAC Standard on 24/7/365 for three years and the (2) 12AX7 and (2) 12AU7 were fine, Plus the case was sealed. (and still only became a little warm._ so if a tube exploded. it would be kept in the case. All otehr preamps I own have open slots or grillwork, or a plexiglass cage (CJ.)                     
I played music every day all day.                
For the (3) 6DJ8 in my ARC SP-15 I leave it off since I do not use it a lot. I want them to last a long time.                      
For the Conrad Johnson ACT2 which is a oven when on, I turn it off due to the heat.                        
The Audible Illusions Modulus 3A I turn off to save the tubes also.    
                       

So sorry to hear about Ron Sheldon. I hope he improves

would agree with hifiman, the 12au7 series of tubes should last a very long time; not like power tubes where you'll get a few years out of them and they need to be  replaced.

would also suggest not to rule out the industrial grade tubes such as the 6189, and 5814. The GE triple mica 5814 black plate is a fine sounding tube. Many of these carried the 5 star rating, for broadcast use. similar to RCA's Command line of tubes.