Excuse me, I looked at the post with the voltage readings table.
I see a power supply problem. One that explains your initial problem with the lack of DC offset voltage adjustment range.
AWR-101 pin 14 should be -51.5 volts DC and is -13.7 volts DC.
It feeds the AWH-050 power amp pins 1,2, 16 & 17 for the input portion of the amp, AS WELL AS EACH CHANNEL'S INDIVIDUAL offset voltage adjustment source network, which EXPECTS to have +36.5v DC AND -51.5v DC. This network has a pair of ground referenced voltage dividers, one (62K, 1K) for the +36.5v DC that makes +0.52v DC , and one (82K, DIODE, 6.8K) for the -51.5v DC that makes -4v DC. Those two resultant voltages go to a divider (100k, 10k pot, 9.1k) that feeds R19 (or R20 for the other channel) into the power amp's input section. THE VR1 and VR2 pots should have +0.180v DC at one end and -0.220v DC at the other end.
With just -13.7v instead of -51.5 these voltages will be messed up - probably not going negative enough across the pot.
AWR-101 pin 15 voltage also has a problem, it should be -19 volts DC and is -4.28 v DC,
HOWEVER this pin is dependent upon pin 14's -51.5 (-13.7) , so it should be ok when pin 14's voltage is corrected.
This pin 14 voltage only feeds the phono preamp awf-011 pin 2 so it's not affecting your offset.
Troubleshooting that regulator:
AWR-101 pin 13 shows the correct -13v from the negative regulator's reference voltage OF -13v dc, meaning a lot is working.
I suspect Q2 2sb507p may be damaged. Voltage readings to ground of Q2 and Q4 are needed.
I see a power supply problem. One that explains your initial problem with the lack of DC offset voltage adjustment range.
AWR-101 pin 14 should be -51.5 volts DC and is -13.7 volts DC.
It feeds the AWH-050 power amp pins 1,2, 16 & 17 for the input portion of the amp, AS WELL AS EACH CHANNEL'S INDIVIDUAL offset voltage adjustment source network, which EXPECTS to have +36.5v DC AND -51.5v DC. This network has a pair of ground referenced voltage dividers, one (62K, 1K) for the +36.5v DC that makes +0.52v DC , and one (82K, DIODE, 6.8K) for the -51.5v DC that makes -4v DC. Those two resultant voltages go to a divider (100k, 10k pot, 9.1k) that feeds R19 (or R20 for the other channel) into the power amp's input section. THE VR1 and VR2 pots should have +0.180v DC at one end and -0.220v DC at the other end.
With just -13.7v instead of -51.5 these voltages will be messed up - probably not going negative enough across the pot.
AWR-101 pin 15 voltage also has a problem, it should be -19 volts DC and is -4.28 v DC,
HOWEVER this pin is dependent upon pin 14's -51.5 (-13.7) , so it should be ok when pin 14's voltage is corrected.
This pin 14 voltage only feeds the phono preamp awf-011 pin 2 so it's not affecting your offset.
Troubleshooting that regulator:
AWR-101 pin 13 shows the correct -13v from the negative regulator's reference voltage OF -13v dc, meaning a lot is working.
I suspect Q2 2sb507p may be damaged. Voltage readings to ground of Q2 and Q4 are needed.