Gain is actually a funcion of the amplifier. The guitar has no part in it, except to supply a low-voltage signal for the amp to work with. Gain is defined as the ratio of unit output per unit input: G = o/i. A high-gain stage in the preamp simply increases the signal voltage more than a low-gain stage. A higher signal voltage out of the preamp is more likely to overdrive the power section, producing more distortion. Most preamps have a multi-tube gain stage that allows one tube to overdrive another tube, giving a distorted sound even before the signal reaches the power stage. This is what happens in the Marshall Valvestate amps. Most of the distortion is produced in the preamp, allowing the output level (Master volume) of the power stage to be adjusted without affecting the amount of distortion. Whether you like preamp distortion better than power amp distortion is a matter of taste.