Tube Preamps Should Have This Feature

The Schiit  Freya + will do just that, see below:

Mike

Whisper-Silent Tube Stage—With Tube Shutdown

''Now, you can enjoy a tube preamp without the hiss and hum of classic designs. Freya+ offers super-quiet tube modes with DC heaters and semi-circlotron, noise-cancelling output stages. Better yet, the tubes turn off when you’re not using them! That’s right, both the tube heater and high voltage rails go away when not in use, so you can preserve lifetime of costly tubes.'' 

I have literally never hesitated to turn on my system. If you can’t afford new tubes, spend less on groceries.

and obtw, the architecture of a passive pre it totally different.  You are asking for 2 totally different components.

"casual listening for a short time"

Your system is going to sound darn good 2 minutes after you turn it on. Try it. Play something favorite, involving I bet. Wait 30 minutes, play again. How much ’More’ involving?

The practice of waiting 20, 30 minutes, (I do it) we convince ourselves, for me it’s why not, but I go cold often enough? Someone coming over, oh helllll yes, warm it up!!!!!!!!!!!

@dweller 

I think that you're worrying way too much about tube wear. Life is short; just use it and be happy.

Agree that life’s too short to stress about such things, especially with regard to a preamp where tubes tend to last pretty long and are relatively cheap to replace.  Music soothes the soul even if it’s just for 30 minutes, so just do it!