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

Timlub, caps unused for very long time, as I mentioned before, loose their rated breakdown voltage, but it takes very long time.  Caps might explode because of this, but often when caps are getting very old and dry the avalanche effect (High ESR=heat, heat=higher ESR), can build-up pressure and explode the cap. 

I'm not sure if charged capacitor has more pressure inside, but I'm pretty sure that warm capacitor does, so it might improve the seal, I agree.  I've seen few capacitors leaking, but it was decades ago when capacitors were not up to current design, materials and production standards.

Also, there were some very poor quality electrolytic caps made in China few years ago exploding in computer power supplies.  I would not be surprised to find them also leaking.  A lot of parts are made in China, including pretty much all ceramic caps and quality is excellent, but some Chinese companies might take shortcuts.  I would feel safe with electrolytic caps as long as the are branded by well known companies.  That way my home state "Illinois Capacitor" (now part of Cornell Dubilier) might have factories in China still being very reputable company.

Czarivey, same here

I had a 1960 Fender Deluxe guitar amp that had caps that only lasted 45 years…damn…replaced them after noting some leakage (sounded fine with the leaky caps) and afterwards the amp sounded exactly the same. Lesson learned.

Damn Wolf, cream Tolex with Oxblood grill cloth? Great, great guitar amp.

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.