There are arguments for and against leaving your amp on or turning it off. Here are a few aspects to consider.
Power consumption costs It is hard to argue with the thought that power costs money. Each month we receive our electricity bill and it always amazes me how much power we consume and how expensive it is. If power costs on average 25 cents per kilowatt hour in New Zealand and the Perreaux 350 power amplifier consumes 100w when in idle, the weekly cost of leaving a 350 on would be approx $4.20 and around $18-00 per month. Power costs tend to add up over time, so if you are not so financially well off, then this may be a factor to consider.
Wear on components An amplifier may have 1000 components inside it. Each one of these components will wear out over time. So which components are going to degrade first? Our experience suggests that it is the electrolytic capacitors which bear the brunt of the stress and are first to show signs of deterioration. If you are concerned about this, then turning off your amp is a sure fire method of reducing wear and tear on them. If one or more of the mains smoothing capacitors ruptures and leaks over time, it can cause enormous damage to the amplifier internals.
Component drift over time Small signal stage devices also can drift out of spec over the years and the resultant sound may not be as sharp as when you first purchased your amp. I guess if you wanted to reduce this effect, then turning your amp off when not in use could extend this within spec period out significantly.
Ahh yes there are always two sides to any story. So here are a few thoughts on why you may wish to leave your amp on continuously.
Power consumption costs so it costs you $18-00 a month to leave your amp on continuously. Big deal .... it is a pittance compared to what it cost to buy in the first place.
Thermal stability At 35 KG, a really big amp like the Perreaux 350 can take a few hours to come up temp and become thermally stable. Now isn't it also a thought that the majority of damage caused to electronic devices (and also cars) is during initial turn on. The alternate method of turning amps on and off causes heat cyclng which can cause serious concequences to your amp.
Inrush currents the inrush currents can be substantial at moment of turn on, which can in turn cause issues.
Voltage stress Imagine a poor capacitor sitting fully discharged and then all of a sudden, hello it is subject to mains voltage. Surely this has to be stressful for electronic components to be subjected to this on a daily basis over a period of years.
As you can see there are pros and cons. We can't say conclusively which is better.
From experience, we feel that the benefits of leaving your amp (s) continuously on tends to outweigh turning them on and off all the time.