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

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.

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.

I have a set of adapters to use my preamp with my speakers in case I don't have a power amp to use at the time. The preamp (MP-1) will play my speakers with about the same volume a type 45 SET. But the tubes aren't run that hard and last for years without any degradation of the circuit boards even after decades of use.

What you are saying here suggests that such is a different matter with ARC preamps. But I think if that were really a serious problem we would have heard about it by now! Preamps bursting into flame is a pretty unusual thing and the age of the Internet has been with us for 24 years!! And no such reports. So I think we can regard  this as a 'theoretical' (or perhaps more accurately as a Red Herring) rather than an 'actual' and only applying to certain ARCs, which, if true says more about them than tube preamps in general.

But I do find myself doubtful about this idea- I have friends that work at or have worked at ARC for many years (I live in the same town and knew them before they went to work at ARC) and none of them have reported anything like this (and they would have if they'd heard about it) so I just don't think this is a concern. 

I follow the on once, off once, per day.

There are only so many electrons in a vacuum tube. The longer you burn the tubes, the shorter their lives are going to be. You can expect to wring about 10k hours from pre-amp and driver tubes, and about 3k hours from power tubes. An important issue is how you bias your power tubes. That will have a major effect on their longevity. Yes, a lower bias in a class A/B will create a lower transition to Class B, but you probably won't hear it at lower volumes. This is not an issue with signal tubes used as drivers and in pre-amps: they are running full-tilt all the time regardless of volume and tone settings. The volts applied to them will affect their lifespan, but that's not something you can control from the front panel. 

Well Atma is spot on as usual.  It depends on design. The tube preamp I have owned for years, TRL, ran barely warm and was a better climate for the parts inside.  The designer always told me to keep it on unless traveling for days etc...

This actually led to better sound, happy internal parts, and longer tube life according to the designer.  

I have never heard of a tube preamp catching fire and Atma's comments cannot be improved on.