Some power amp info scraped from the net:
The SYN-20IR offers unique controls for STRUCTURE and REACTANCE that tailor the response of the amp's all-analog power section. The revolutionary STRUCTURE feature optimizes the head for any channel by offering three distinct power amp configurations and feedback which are assignable to each of the amp’s channels. The STRUTURE feature significantly alters the overall contour and feel of the SYN-20IR.
There are three Structure modes to choose from:
• Mode A: Balanced negative feedback for a clear yet punchy sound
• Mode B: Vintage style phase inverter with relaxed mids and a bright, shimmering high end.
• Mode C: Modern HiFi style phase inverter and low negative feedback delivers a modern, tight feel with solid lows and a darker high end.
REACTANCE is similar to a presence knob, but with a dynamic response and variable phase inverter which simultaneously shapes the middle and treble frequencies. This powerful control helps tailor the sound to your chosen module and Structure setting.