The amplifier (AMP) is a 2 stage Class A discrete design using 3 pairs of hand matched transistors. Envelope is usually applied to ST1/1.  ST1/2 goes thru an attenuverter and can be used for envelope or modulation.  ST2/1 goes to the 2nd stage, and is typically used for volume control.  ST2/2 goes thru an attenuverter and is typically used for modulation.
The AMP, the 5089, and the 3003 are Class A circuits. They draw more current from the +12 rail than from the -12 volt rail.  If the system power supply is under-sized, hum or buzz may be heard. In that case, larger filter capacitors and/or a larger power transformer will be needed.  The 4075 draws equally from both rails, and is more immune to hum.