This doesn't sound right. How did you test?
I followed the measurement procedure, sending the different sine waves and measuring on output XLR betwenn pins 2 and 3.
I used the same 500 extender, same rack slot.
For what settings and pot position?
input signal = sine 1kHz, 0.4VAC TAPE SAT on left 9.2V Unit 1 10,6 V Unit 2 9,3V
(on input XLR between pins 2&3) TAPE SAT on center 7.4V Unit 1 8,27V Unit 2 7,4V
BIAS on right TAPE SAT on right 7.1V Unit 1 7,56V Unit 2 7,08 V
Input signal = sine 40 Hz – 0.25VAC SPEED on 7.5' 9.4V Unit 1 10,5V Unit 2 9,9V
TAPE SAT on left SPEED on 15' 8.5V Unit 1 9,7V Unit 2 9,10V
BIAS on center SPEED on 30' 7.5V Unit 1 8,5V Unit 2 7,95V
Input signal = sine 60 Hz – 0.25VAC SPEED on 7.5' 7.5V Unit 1 8V Unit 2 7,7V
TAPE SAT on left SPEED on 15' 9.3V Unit 1 10,4V Unit 2 10,06V
BIAS on center SPEED on 30' 8.7V Unit 1 9,9V Unti 2 9,46V
Input signal = sine 80 Hz – 0.25VAC SPEED on 7.5' 5.4V Unit 1 5,7V Unit 2 5,5V
TAPE SAT on left SPEED on 15' 7.9V Unit 1 8,5V Unit 2 8,1V
BIAS on center SPEED on 30' 9.3V Unit 1 10,6V Unit 2 10V
Both pots on 10 BIAS on right 6.1V Unit 1 7V Unit 2 6,75V
Input signal = sine 10kHz – 0.25VAC SPEED on 7.5' 2.5V Unit 1 1,34V Unit 2 1,32V
TAPE SAT on left SPEED on 15' 4.1V Unit 1 2,3V Unit 2 2,2V
BIAS on left SPEED on 30' 5.1V Unit 1 2,7V Unit 2 2,7V
Both pots on 10 SPEED on 7.5' and BIAS on right 5.1V Unit 1 2,75 Unit 2 2,7V
Thanks !