Tube sound is not about warmth. It's about correct presentation.

There is no "warm" tube sound. Tubes get warm or even hot and some dimwit reviewer probably used the term "warm" or "warmth" and it went from there. If you believe a sound can be warm or cold, you're like Ralph Wiggum when he said, "It tastes like burning."

@mrskeptic (What’s in a name?) To me ElL-34’s in most amps sound warm. Maybe warm means not fast. Other tubes like the KT-150’s are not as warm (at least in my amp).. There’s something in the micro timing or thier musicality (sound rendering). Last but not least tubes are time proven analog. And many love to see ‘em glow.

It's about added distortion. The end.

How much distortion is required for 'correct presentation'? 

I am of the opinion that Benchmarks class G amplification sounds to sterile, at least when I auditioned it some time back, but I am also a valve fan and enjoy my distortion like Nelson Pass.