I have no idea what a 'specialist on "warmth"' is, but I know what 'warm' and warmth' mean.
It's a characteristic defined by frequency response, with the upper-bass and lower-midrange frequencies (specifically, from around 100Hz to maybe 500 Hz) higher than the rest of the band. It's NOT louder midrange frequencies--that's forwardness, and it's NOT quieter high frequencies--that's darkness, the opposite of brightness.
Many of us like speaker systems that are a bit warm because they reinforce what my friend calls the 'power' region of the orchestra, the frequencies where so many instruments--cellos, trombones, French horns, tympany, etc.--have their fundamental frequencies. But warmth quickly turns into thickness, and I believe not many of of like 'thick' sound.
I hope this helps.
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