Amps and Preamps has standby, but should I turn them completely off?

The reason electrolytics need to be brought up slowly after a long sleep is because they will exhibit leakage current which multiplied by whatever voltage will produce some watts of heat. I dont think ESR is the main factor in caps exploding. It is best to form caps with a constant current which is sized to the cap. You can then watch a 350 volt cap charge rapidly to 200, slow down and grow to 350 in several minutes and then past its rated voltage to its surge voltage where things stop. Good caps can have this top voltage be 20-50% higher than the rated voltage. What voltage a cap was made to is not always what is on the label. I talked to an old timer at P R Mallory and got the good scoop on all this. Its the water in the caps that turns to steam that causes them to explode. Its also this water that dries over time and increases ESR. But even is ESR doubles I would not think that is a big contributor to heat. 

When you form a cap you are actually rebuilding the aluminum oxide layer to its original level. BTW this is why high voltage caps cost more because they need more plating time in the bath when being made. The plating process slows and cuts itself off at about 600 volts. That's why you dont see higher voltage caps. The surge has to be about 20-30% higher than the operating voltage for several reasons. 

I use a current of about 1 mA per 100 uf. depending on the physical size and the voltage I am going to. My examples are higher voltage because I make tube amps. 

Guitar amp power is usually measured at 5% THD. At low powers they can be quite clean. The older brown face Fenders were really clean. This was before Hendrix and the craze for overdrive, which is just another name for clipping/limiting, turning everything into square waves. If you know how a limiter works in an FM tuner, thats what they did.