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 !