Always ON?

Tubes I turn on 1/2 hour to 1 hour before listening.

I leave my phono preamp on all the time.

I have a Sony XA777ES that I leave on all of the time because it has been modded with black gate capacitors, and they take hours to warm up. I'm talking about 12+ hours.

I don't know what's best for the equipment, but I know leaving them on sounds the best.

Change in sound over time is common in speakers. The problem is acute with lower cost drive units.

Heating of the voice coils while playing music causes thermal compression. This reduces the dynamic range audibly and can cause drifts in the way the crossover performs in passive designs. It is partly the reason for blowing speakers at parties...volume goes up until it sounds good and loud....after half an hour the speaker voice coil gets very hot....sound levels/dynamic range drops due to rising coil resistance...so volume gets cranked up again to compensate...speaker coils get even hotter...=> eventually leading to failure even though sound levels may be no higher than when you first switched it on!

If you find that your system sounds better after half an hour of playing, it could be that your speaker manufacturer may have designed for your speakers to run optimally at a certain voice coil temperature (when warmed up)....or you may simply not like the uncompressed bigger dynamics when the voice coils are cold.

Large profesional studio monitors deliver some of the biggest dynamics available on the market but people who have only been exposed to consumer audio often find this too startling and not to their taste.

I leave my SS int amp (Plinius 9200) on 24x7 and I've found that it clearly sounds better vs. not leaving it on. My TT and tube phono stage take two LP sides to warm up, and I can clearly hear further improvement through 3 LP sides.

I also like to turn the tuner on and at volume for 30 min to an hour prior to LP listening because I've found that it helps to warm up the system with some current flow rather than just being "on" -- perhaps this is speaker warm-up as described by Shadorne above, but I think its the amp as well.