Here is what the owner's manual says:
Magazines and product literature have both contributed to a great deal of poorly informed
discussion of class A operation over the years. Unfortunately, the thermal
management problems of true class A operation in a high-current output
stage are severe, and introduce serious sonic compromises of their own. For this
reason, the output stage of your new amplifier are not class-A biased in a traditional
fashion. (All voltage gain stages of the 300-series amplifiers are biased to
operate in a full class A mode in order to keep the active devices safely within
their most linear, distortion-free range at all times.)
The 300-series amplifiers use a proprietary adaptive biasing scheme developed
by Madrigal (and first introduced in the Nº 33 Reference Monaural Amplifiers)
that delivers the sonic advantages of a Class A output stage without incurring the
substantial inefficiencies and consequent thermal problems of pure Class A operation.
Uniquely, this adaptive biasing scheme never allows the output devices
themselves to be reverse-biased. This approach results in greatly reduced dynamic
distortions, and a sweeter sound that exhibits a greater sense of ease at all
volume levels
