[Thanasis]

I think it is a placebo effect to think that at low volumes a Tube Amp will give you any benefits or warmth of the tube....
(Some decades ago I remember the sweet sound of my grandmom's tube radio and I long that era)
 
If you don't drive the Tube to its limits it will not give you the effect.
 
So I think, a Proffessional Compressor is the ideal tool to keep the Tube to its limits (Overdrive the tube a bit) to deliver what a tube has to deliver even at low volumes !!!.....
 
It is just an idea.

 

[Thanasis]

Ok, now,

If the amp has 2 level of amplification (Gain and Power tubes) then the gain serve as a compressor.... So then you fell safe !!!