Sag is mostly a result of the higher dynamic current flow of the power tubes. If the amp has less power supply filtering and/or uses a tube rectifier, the sudden onrush of current caused by big signal transients causes the B+ voltage in the amp to be pulled down or "sag". It's sort of like how your lights in your house might dim for a split second when a big electric heater or motor kicks on. The more the B+ voltage sags, the more the amp tends to compress and break up. Having no sag at all would make for a much more stiff and clean sounding power amp section, so it kind of makes sense that turning it all the way down disables the power amp modeling. In reality all amps sag at least a tiny amount since there is no such thing as a perfect current source.