I second Slowhand's comments. This from the AI Modulus 3A manual:
"Main Out #2 is the same Main Out #1 but includes a 470 Ohm resistor in series with the output. This "higher impedance" corrects for anomalies, i.e., "tube glare" that are exhibited in some brands of tubes."
"Main Out #2 is the same Main Out #1 but includes a 470 Ohm resistor in series with the output. This "higher impedance" corrects for anomalies, i.e., "tube glare" that are exhibited in some brands of tubes."