Is it a OK to leave tube pre-amps powede up 24/7

Short answer is NO with any model. Not only it's unsafe, but it's useless waste of tubes and other circuit components as well. It's a huge tax on component longevity. 

1. tubes dissipate heat that dries out electrolytic caps. 
2. occasionally blown tube can blow other circuit components or even set circuit board on fire. 
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tubes worm up a lot faster than transistors and caps 'fill up' with charge nearly instant. so what's the point running it on even throughout the night?
turn on for 10...20min before playing music and enjoy. 

1. transistors in most class A/B and class D amps don't and there are many class A/B amps that don't run hot.
2. I've repaired tube equipment with burned circuit board around blown tube more than once and for me one is too many anyways.

Old saying says -- No problem till the SHTF. Right?

@audiotomb,
 I understand a concern of vintage NOS tube dealer, but at the same time you never know if one will be running them 24/7 non-stop, but according to most of contributor's here with "deep experience and knowledge", some, as turn out, definitely will...

Well, some  ARC tube preamps can power up a pair of small speakers if connected to the output to significant SPL levels.
That is to say that signal tubes there are driven rough almost just like power tubes.
PCB might not catch fire, but it might burn 'foil side' that in places will need to be either rebuilt or point-to-point wired. 
Once again, the possibility of failed signal tube to affect other circuit components as well as PCB is there.

Folks,

It's not even about tubes, It's about TEMPERATURE and the length of exposure to such temperatures. Speaking of ARC tube preamps YES they run hot! A faulty DC capacitor can cause excessive bias current and destructive heat that can cause PCB go to flames. 

As I've mentioned prior, one is too many, but repaired more than one. Running units 24/7 in simple term mean you don't give your circuit elements a 'recoil' time to recover after applied heat. Shortening life and paying extra for the idle time. That's where simple math shoves all the science behind.