Fire when you speak but ain't no warmth in your vocals
Lupe is referring to how many rappers claim that they “spit fire”, meaning that everything they rap is hot/good. Though he plays on the antonyms warm and cool by pronouncing the line as “ain’t no warmth in your vo-cools”.
Fire produces heat, which provides warmth.
He is saying that even though what you rap is “fire” there is no “warmth” in anything you say. Everything is about drugs, sex, money and being a thug, instead of showing some support for other rappers and speaking on something positive.
In the previous line he alludes to the tube. The tube being a microphone preamp. Certain Tubes/Preamps are known for giving a warmth character to the sound or vocals when used. Hence if you don’t have the the tube, the warmth is gone.
Lupe commends Kanye on being able to speak/inspire/preach, but he also criticizes ‘Ye of being cruel and rather cold to the hip hop community and people in general
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