This config seems to be OK so far - much more testing to be done, though.
Specifically, I need to test telephone conversations, teacher-as-source and cassette-as-source.
===== Duo.cfg =====
[RECORD VOLUME]
LINEIN=5000
MIC=90000
CD=56071
AUX=0
WAVEOUT=0
DIGITAL=58339
ANALOG=58339

[PLAY VOLUME]
LINEIN=15000
MIC=5000
CD=26025
MAIN=21000
WAVEOUT=20000
AUX=0

[PLAY MUTE]
LINEIN=0
MIC=0
CD=0
WAVEOUT=0
AUX=0
MAIN=0

[REC MUTE]
LINEIN=0
MIC=0
CD=0
WAVEOUT=0
AUX=0

[REC SELECT]
LINEIN=0
MIC=0
CD=1
ANALOG=0
AUX=-1
DIGITAL=0

[ADVANCED]
MICBOOST=1

[CD]
DIGITAL=0