Actually, I have heard and read the opposite, which is that tube gear is fine to turn off and on (assuming you provide a bit of warm up time before listening sessions), but that solid state gear needs a couple of hours, at least, to sound its best, which why some people leave their solid state gear on 24/7.
However, to be honest, I tend to turn off my equipment between listening sessions, mostly because I rarely listen from day to day, and it seems to be a waste of energy (hence money) to leave it on 24/7.
That being said, my solid state preamp, an Ayre K-1xe, is always on, as there is no on/off switch actually, (it is in stand-by mode), and basically, I do is unmute it to turn it on. But preamps tend to draw very little current, so there is no real problem with leaving them on.
My hybrid amps, Lamm M2.1 monoblocks, (which draw a lot of current), I leave off until I need them. Then I turn them on, and let them warm up for 20-30 minutes before I start listening. The sound is very good after half an hour of warmup, but does get slightly better after another half an hour or so. After 45 minutes to an hour, the sound does not seem to change much.
My two cents worth anyway.