Hardware modifications for getting standard line levels in/out (-10dB),
instead of the original microphone in/headphone out. These modifications
are described in your Cubase Audio handbook as well, but there is a slight
change from that to get the true line levels.
All the components are surface mounted resistors and capacitors, so you should NOT do the modifications if you're not absolutely certain that you are able to do it.
REMOVE THE BASSBOOST
Change C58, C59 from 100pF to 1000pF
Change R123, R130 from 10kohm to 15kohm
Change R59, R68 from 100kohm to 4.7kohm
Change C95, C90 from 0.033uF to 1kohm resistor
LOWER THE HEADPHONE AMPLIFIER'S GAIN
Put a 10kohm resistor between pin 2 of U21 (@ R119) and AGND (@ R111).
LOWER THE INPUT AMPLIFICATION TO -10DB
Remove R56, R57, R90, R92, C21, C81
Change R66, R87 from 100kohm to 1.8kohm
Put a 470pF capacitor in parallel with R67, R88
Put a 100pF capacitor in parallel with R66, R87
Enjoy the standard levels you've got now...