Technically speaking, what is the "warm sound"?

I find that you can easily go to sleep with a tube system yet my solid state system tends to get me to turn it off before nodding off.

So far I tend to like my 7868 output tubes system better then my el 34 systems based on limited listening with different speakers.

 

I believe this is the key factor.

My guess is that a limited slew rate not only changes the source sound but also modifies the dynamic distortion components, as overshoot is limited.

I tend to believe that gear should provide a lean sound and the room, including artificial ambiance, should provide the warmth.

 

Tube rectifiers (for what little I know) also add some 'sag' to the power supply under high current demand, which I believe SS rectifiers don't.

This probably accounts for some kind of compression on the output signal.