I've been wracking my brain for some time now with this question. I have a theory that the environment inside the tube as the sound is converted to voltage and amplified by the tube that harmonics (or something) are being added to what eventually is emitted from the speaker as sound. I'm thinking that the electrons as they are emitted from the cathode and pass through the grid and hit the plate inside a glass tube are being supplemented with harmonics relative to the glass surroundings and the metal filament ( the grid) and I'm wondering if amp models account for this if not I'm hoping they could.