When to Poweroff amps and preamps

Always off, unplugged from power.
 When I will listen, I will power on amps, pre and disc player 24 hours before I listen.
 All solid state gear.  
 Goes for both my systems, they sit dormant and unplugged until I want to have a listening couch sit down.  
I may go months without listening, and when storms roll in, they are already unplugged.  Vacations, when not in use, they are unplugged and not in use.

as mentioned,  if listening on a fri & sat night, I will power all on, and leave on for the whole day and afternoon starting on Thursday, to allow them to warm up.

  Amps on first and off last has never failed me!

  They always sound better after a good 24hrs+ of being powered on and sitting idle!
 Been doing this since I was a wee mite in 1985 with my first separates system!
always took care of my gear, as I payed for it w my own money!
You may do whatever you feel is right, and works for you.


  
    

You have to turn off tube amps but solid-state amps you can and SHOULD leave those on all the time!

I’ve never had a SS amp fail due due to on/off. Every device is unique. If you can hear a difference with yours and don’t mind paying for unused power, by all means keep them on 24/7. You better have real surge filter/suppression in series for the enivitable electrical storms.
Of course tubes have finite life spans. You gotta pay to play.
Check your stuff with a Klil A Watt.
Most "stand by" uses 90% of the kWh for a few seconds of faster play.
No audible difference with mine.
I even unplug the power sucking cable box off nightly via a timed outlet.

We all have our views and opinions, including technicians and engineers.  Depending on the design of the product on/off can play a role for one and a different role for another.  I have tube gear for phono stage and preamp, SS for power amp.  Everything is connected to a quality AC conditioner with surge protection.  I turn on about 30 minutes prior to listening and turn off when done.  Heat is the enemy of electronics and my preference is to minimize heat exposure to my electronics.  Tubes have a life span far less than SS gear so it's a no-brainer to turn off that gear after use.  For simplicity sake, the SS amp is turned off as well.
That said, even with the 30 minute warm up prior to listening, there is ALWAYS a period of running time in which SQ gets better.  I find that about 30 minutes in of listening and the imaging improves, bass gets tighter, and soundstage gets wider.  About 45 to 60 minutes and all improves still again.  Listening and enjoying the slow improvements is actually rather fun.  Personally, I prefer to do things this way rather than have heat prematurely deteriorate the electronics.  As stated, we all have our views and opinions; these are mine, yours may differ.