Quote Originally Posted by Eric
So I take that to mean that most of the distortion/warmth/harmonic activity in clean tones is coming from the preamp tubes? Those are usually 12AX7, correct?
It depends on how the amp is engineered. Leo Fender and later CBS, spent many years trying to get Fender amps loud and clean. It can be either the preamp tubes or the power amp tubes, or it can be a combination.

It depends on how many gain stages are built into the amp. Little amps like the Champ, have two gain stages, i.e., one 12AX7 and the preamp tube doesn't distort at all. All the breakup comes from pushing the 6V6 tube very hard. The only way to get the preamp to distort is to push it with a clean booster or OD pedal.

Classic Marshall Plexi amps, have three gain stages and a phase inverter i.e., three 12AX7s, before the EL34 power tubes. Some people install master volumes to get the amps to distort at lower volumes. But the classic plexi sound is distorted sound pushing the EL34 tubes hard. Those classic plexis do sound good at low volumes, with no apparent distortion happening, it's just that most guitar players don't play them that way.