In Amps! The Other Half of Rock 'n' Roll Ritchie Fleigler wrote that "...old amps derive much of their sound from their (sometimes intentionally) under-engineered transformers and sagging voltage supplies."
Voltage or power supply sag is a major contributor to an amp's organic sound. The heart of the amp, the power supply is the foundation where the amp's soul is born.
When a traditional amp's power supply is cranked, the strings come along, technically producing a lot of voltage sag. A very slight delay and slight variation of volume cause notes to bloom or blossom at your touch.
When a traditional amp is turned down, or deliberately designed to produce little or no sag, the guitar strings behave as if they are directly connected to the speaker; snappy and immediate. You can definitely have too much sag, but zero sag is crystally clean, tight and great for bass amps.
Manufacturers have been working on the wattage/volume issue for years. You'll find a detailed explanation of these solutions at the wattage control page.
And yet, manufacturers haven't changed the power supply. Virtually all amplifiers made today use that same under-engineered power supply Mr. Fliegler refers to. Leo Fender borrowed this design over 60 years ago from the RCA Designer's Handbook, a manual initially pubished in the 1940s by RCA to help engineers and technicians design equipment around their vacuum tubes.
These 70 year old power supplies:
Dave Zimmerman got the idea for the Sag Circuit during graduate school while reading a Guitar Player interview with one of his guitar heros, Neil Young. Mr. Young explained that to get his amazing tone, he was creating conditions that were pushing the power amp and power supply well beyond its normal limits. This interview helped Dave realize that if the power amp could be controlled, the amp's organic tone could be controlled--at any volume--in very new and exciting ways.
Applying his understanding of nonlinear analysis (or chaos theory) to guitar amplifier design, Dave started working on prototypes.
On June 3, 1997, the Sag Circuit's first patent #5,635,872 was issued after an impressive 19 month application period (with only one other patent cited--#4,797,633, 1/89, Humphrey). The continuation-in-part (patent #5,909,145, an elaboration on the first patent) was issued on June, 1999, again citing Humphrey as the only other patent that can be remotely compared to the Sag Circuit. The patents are theoretical in nature, covering thousands of implementations.
The Sag Circuit gives you more control over your musical instrument than any other power supply possibly could.