Tube preamp, Leave On Always or ON/OFF?

I'd turn it off. Wastes tube life. However, some tube preamps run the tubes at very low power, so that can litterally last a long time. Some questions to ask yourself, do you notice a difference in sound quality if left on for days at a time? Or after a hour or two, things are warmed up and sound fine? Are you running stock tubes that are cheap to replace? Or expensive NOS? Turning tubes on and off a lot can reduce life span.

What I do is on weekends, when I listen a lot, leave it on. During the week, goes on and off...

I substantially agree with Jfrech. Mine is a tradeoff. I use a very difficult to obtain tube with a 10,000 hours usable life. This is about a year and a half when always on. It takes my preamp about 2 hours to reach its peak, and I typically use it for 3 hours each night and about 5 hours each weekend. Thus I use about 41 hours per week turning it on and off, rather than 168 hours leaving it on. My tubes thus will last about 5 years.

If you have the dough to replace tubes often, then leave it ON; otherwise leave it OFF when not in use.

ALWAYS ON. Solid state or tube. Always.

I never turn mine off. It's an Audible Illusions M3A. It's known as a "tube eater". I had the same tubes in it (Amperex JAN) for 5 years. The JAN's are still good! I just took them out to listen to different tubes.

Think about it. Professional studios leave everything on constantly. Nothing heats up then cools down then heats up .... These expansions (heat) and contractions (cool) cause a shorter life.

To test this, leave a light bulb on. Isolate it from all vibrations. (You never thought you'd be using cones, cork, sorbothane, ... for that, did you?) That light bulb will last longer than any other light bulb from the same pack.