Why warm amps sound better.

Ok, I just turned on my system for the weekend after about 3 days turned off. 
The opening guitar of UFO's Obsession just seemed tinny, edgy.

i know warmed up amps /preamps do sound better, the capacitors are warmed up and sound their best........why?

i had left my stereo on for about 2 days last weekend, and after 24 hours, I was just sitting having a beer, relaxing listening to Metal Church, and noticed my eyes drifting closed, I was not tired, but relaxing, my wife put on her fav cd, Def Leppard's hysteria, the sound was very good, much better than a cold start/listening session.  The guitar tone was full of emotion, soul, just seemed to flow from the speakers to my ears in a magical telepathic way :) I know this is boring, jus stick it out, will ya?

im not going to plug my gear, I just was curious why when amps get nice and toasty, they can sound completely different, almost realistic, with a sound of rapture. 

Its a night and day difference between cold start, and leaving on for a couple days, it's really a huge difference in sound. 

Why do warm capacitors sound so different from cold ones. 
I have tried this with my preamp/amps/cd player. Each one started cold on a different day while the other components were left on.  Each time the sound was harsh, not bad, but noticeable. The amps being cold was the biggest sound change. Followed by the CD player. 

Please explain, I need to kick the volume up a few db's. Thank you