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
I have no idea. I’ve heard this much worse in tube gear than SS though. I listened recently to an ARC integrated amp, which kind of acted like a pendulum until it was fully warmed up.  At first, cold it was OK. While warming up it got really hard, and 20 minutes later it came back and sounded great.

At those power consumptions and tube costs though I could never consider keeping it on all the time.

Went Class D.
I know, I was just thinking about the worst examples to help tease out the "why."

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.

+1 timlub. However I turn mine off after a listening session. Then turn it on about 2 hrs before next listening session. Mysteries of past life experiences I am sure. Then I am not sure about much. There are lots of things that can be measured and no doubt a lot that can't. But the most important thing is this topic keeps us confused. Me anyway.
Yup, sunfire is a/b. Doesn't it switch to class H ? 
Downconverter goes into class h? 

This thread is going not not where I wanted :)

This is hilarious. I'm 49 years old and both my parents recently passed away (okay, that's not funny). I was packing up stuff at their house and found about 50 of my old albums. One of them was Metal Church (the first release). Wow the memories that brought back.
I was jamming "the dark". Great band! I still have most of their lp’s.
Among the thousand lp’s I own.

 Take that lp, and drop a needle on it. It has held up very well
I leave my SS amp on 24/7. After being upgraded and 5 days of shipping time, it took 4 hours to start sounding very good. The manufacturer said it takes around 24 hours before the transformer reaches optimum temp at it's core.
Relaxing listening to Metal Church ,,, kinda lika chillin to overkill ; )'
    yes my gear needs to be on about an hr before it cleans up.Sometimes i flip it all on a cple hrs early and my 20 mnth old changes the plan,only to walk in feel it all juicy warm and flip it off lol
Yup, sunfire is a/b. Doesn't it switch to class H ?
Downconverter goes into class h?
That's right, Sunfire is class A/B. The Carver amps were class G.
I leave my Sunfire turned on 24/7. The sound from the amp at idle for a few hours is clearly better than after a cold-start.

I turn my primary system on about 1 hour prior to a listening session. All electronic parts need to reach thermal equilibrium to operate at proper spec. I believe this to be true even for speaker cones.   
In addition to bias points (operating points) filter caps need to charge up and they charge on an exponential curve. So that last nth degree takes a while. Filter caps are usually electrolytic and electrolytics are funny things, sort of a cross between a real capacitor and a battery (more on the cap side of things though). They are also used as coupling caps in solid state gear and for that reason need some time.

Transistors have a non-linear capacitive element that is inherent in their junctions (which affects the kind of distortion they make). This aspect is affected by heat and voltage (most radios these days are tuned by varactor diodes that take advantage of this effect). That quality of the devices has to stabilize and if the designer did his homework, that happens after a period of time rather than just after the amp is turned on.

IME solid state gear really needs to be on all the time to sound its best, though I have run into to some pieces that actually sounded worse after a day or so.

One might ask why such tiny effects might be so audible and the answer is that it has to do with distortion, and tiny amounts of it at that. But the distortion involved happens to be of a kind to which the ear is actually more sensitive than most test equipment, which is to say that we can measure that the distortion is there in trace amounts but to the ear its pretty obvious. This is because the ear/brain system uses the harmonics that are affected by this distortion (5th harmonic and above) in order to figure out how loud a sound is. So the ear brain system has to be pretty sensitive to do that as our range of hearing is pretty wide from soft to loud.

This is why the tube/transistor thing has been going on for so long BTW. Tubes inherently make less of these higher orders than transistors, which is why they sound smoother.
To add my 2 cents. When I was a kid we left my friend’s Fender Champ guitar amp in the trunk of my hot car for a few hours.

when we plugged it in it sounded better than it ever had.

The BHK 300 amps (dealer alert) have a solid state section meant to stay on all the time and the tube input section is what gets turned on when the front power switch is engaged.

i have even noticed it takes my strain guage phono cart takes some time to warm up and sound better.

Much to what many people believe, SS amps require more warming up than tube amps.  Some SS amps require 2-3 days to yield their full potential.  Additionally,, many amps are never afforded the opportunity to fully break in when purchased new.   First let your system settle in for no less than 1000 hours, then use the mini break in for a few days and never let your system lay idle for over week.  Breaking in is vitally important for your system to find its least resistant grounding path and, believe it or not, this takes lots of time.  When burning in your system, it is not necessary to play music loudly with heavy rock.  Simple classical music at low levels is sufficient but continuous pink noise also works magically.  Play nothing at all and just leave your sytem on to let your grounding system establish itself.  Grounding is something many audiophiles miss or not pay enough attention to robbing and limiting the full potential of one's system.
My SS amps are left on all the time except during vacations, as they need to be on at least 24 hours to sound their best.  I leave my digital source gear on as well.  OTOH, my tubed pre warms up in only about 5 minutes, and so that gets turned on and off. 
My Class A amps warm up very quickly - maybe 30 minutes, after which they don't change sound.  Which is a good thing considering that this sort of amp uses full current at idle and puts it out as heat (they actually put out less heat when making sound and converting some of that idle current to move the speaker drivers)