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

It's the transistors that need to warm up and reach thermal equilibrium so they can maintain the correct output bias while achieving thermal stability  

maybe Almarg or Atmasphere will stop by and explain in depth 

Transistors need to reach their operating temperatures and that's where all the parameters are tuned for each amp. It's been discussed in various posts here especially those about keeping amp on 24/7.
Some amps require longer worm-up time than the others, but within half-hour even class A amps will have enough worm-up time. Not needed to idle turned on for 24 hours.
I have class D amp (Sunfire 300) that only uses 20w of stand-by power so I don't mind keeping it always on and ready. I did not run my prior Bryston 4b-st 24/7. In 15...20min this amp was reaching a correct operating performance. 
Mine take. A wee bit longer than an hour. 
Stereo always sounds better after a 24 hour warm up.  Everything just clicks after about 10 hours. 

The CD player is new, only has maybe 25-30 hours on it.  Maybe that's the culprit. 
I'm one of those odd weirdos that contend that my amp & preamp continued to improve for a month after I turned them on. Now I just leave them on.
I leave my system on 24/7.  Solid state amp and tube pre.  Sounds good as soon as I sit down to listen.  If possible, I'll play a rocking' disc for about a half hour for the very best sound just prior to a listening session.