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

Life of electrolytic caps is shorten by the factor of 2 for each 10degC temperature increase.  It is simply drying out of electrolyte.  Turning amplifier on often produces rush current, but heat amount from that is negligible.  Simply, your amp will last longer when you turn it off.  Also, life of semiconductors does not depend on how often you switch.  The only case I know where semiconductors are getting "tired" are big SCRs (Thyristors) in welding machines.  They go thru some structural changes caused by huge temperature gradients.  It does not apply to small current devices.  In class D amps power Mosfets are switched ON and OFF 500,000 times a second.  The only drawbacks of switching on and off is shorter life of the switch (not important with a few cycles a day) and warm-up time to get the best sound.

Czarivey, life of capacitor is reduced by factor of two for every 10degC increase over temperature that life of capacitor was specified by manufacturer.  For instance capacitor rated 5k hours at 85degC will last 2.5k hours at 95degC 1.25k hours at 105degC etc, but also 10k hours at 75degC, 20k hours at 65degC, 40k hours at 55degC.  At 25degC (room temperature) it would be 320k hours (37 years).  Different manufacturers define end of life differently but often it is 200% of rated ESR.  When capacitor dries out ESR  (equivalent series resistance) increases.  Increase of ESR produces, with electric current, increase in temperature.  Increase in temperature produces increase in ESR leading to avalanche effect and explosion. 

Czarivey,  will you last 40+ years?

Timlub, I'm not sure why keeping them charged stops them from leaking.  Perhaps some capacitors are not properly sealed, to start with, and would eventually leak.  Keeping them on will most likely increase their temperature (since electronics dissipates heat) drying out electrolyte faster, hence thickening it (and reducing capacitors' performance).  Company I work for has a lot of obsolete electrolytic caps in stock,  often 30 years, and I never heard of any of them leaking.  They are unusable, in my opinion, but it is not my decision to toss them out.  Keeping fully charged might extend life of some batteries, but not capacitors AFAIK.  The only negative effect turning electronics off is reduction in breakdown voltage of the aluminum capacitor.  Without voltage electrolyte eats out dielectric - aluminum oxide, reducing breakdown voltage.  Presence of voltage rebuilds dielectric.  We're talking  >5years of keeping gear unpowered.

Czarivey, same here

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.