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.