Before the mass application of post-preamp master volume controls, old amps had to be cranked up to get distortion. The distortion of the power amp is what gave those old amps their defining overdrive characteristics. Modern, master volume, amps rely on preamp distortion more than the power amp to create crunch and chunk without a person’s ears bleeding from the high SPL.
The 3 Channel Dual Rectifier (non-multiwatt) has an extremely cold biased power amp at full power. This allows it to amplify the preamp distortion with a minimum of additional overdrive being created, if any is created at all.