fed a mono signal, as the image always seems wider and less spatially specified. Why? Some of the blame must lie in the disparity between loudspeakers, differences in frequency response, mismatching in crossover components, and different room reflections. And certainly some of the blame must lie in the disparity between linestage and power amplifier channels.
Expanding the width requires cross feeding an inverted phase signal from the opposing channel (ultimately, this signal should be frequency selective). A 100% ratio will completely eliminate the signal common to both channels, resulting in a extreme separation of instruments, so extreme that some instruments will appear far to the outside of the speakers.
A reasonable amount of constriction and expansion might at the 50% mark for both adding and subtracting one channel from the other. How do implement such a control?
Starting with the blending into mono direction, the plan is simple: place a varying amount of resistance between the plates of two grounded-cathode amplifiers. The smaller the resistance, the greater the blend. Hitting the 50% mark requires that the bleed resistor equal the rp in parallel with the plate resistor in a grounded-cathode amplifier with a bypassed cathode resistor.