If you're dealing with a system, or a sound, in which no agreeable balance can seemingly be struck between warm and cool, i.e. a sound that can be made to be either too cool or too warm, but offers little or no real appreciable apparent, or satisfying, 'sweet zone' in the middle, then you have a sound that is said to have little or no "inner warmth".
A system or sound with a good deal of inner warmth will, by definition, sound neutral without also sounding threadbare. As opposed to an overtly warm sound that may be described as 'pleasing', but strictly speaking is neither accurate nor neutral.