When using Duet2, Duet for iOS/Mac/PC, Quartet, and Ensemble Thunderbolt, the dual XLR-1/4" combo jacks are set up so when the input is set to 'Mic' or 'Line' (+4dBu or -10dBV), only the XLR part of the jack will receive signal.
This means that you will need to use an XLR cable or a cable with an XLR adapter connected to it.
The same is true in reverse- when the input is set for 'Instrument', only the 1/4" part of the jack will receive signal. This means that you will need to use a 1/4" cable.
Here are some examples for setting up different types of input sources on Duet, Quartet, and Ensemble Thunderbolt:
Mics:
Connect your Mic using only an XLR cable (no 1/4″ adapters or cables) to the XLR input jacks on your Apogee device.
Launch the Apogee Control app (Apogee Control, Apogee Control 2, Apogee Maestro 2) from your Applications folder, go to the inputs and ‘Analog Level’ drop-down menu for whichever input you’re using to ‘Mic’.
If you’re using a condenser mic that requires phantom power, engage the ’48′ button on the input you’re using.
Gradually bring the input level in the control app up until you get a good level from your input source. Make sure the meter isn’t going into the red.
Guitars, Basses, Keyboards, Hi-Z Mics that have 1/4″ cables, such as harmonica mics:
Connect your Instrument using only a 1/4″ instrument cable to the 1/4″ input jacks. (be sure that you're connecting to the 1/4" inputs on the combo XLR-1/4" jacks and not the 1/4" output jacks)
Launch the Apogee Control app (Apogee Control, Apogee Control 2, Apogee Maestro 2) from your Applications folder, go to the inputs and ‘Analog Level’ drop-down menu for whichever input you’re using to ‘Instrument’.
Gradually bring the input level in the control app up until you get a good level from your input source. Make sure the meter isn’t going into the red.
Sending Line-Level audio to your Apogee device and bypassing it’s mic-pres:
Connect the output from your line-level piece of gear (such as a compressor or external mic-pre) to the XLR input jacks. If your Line-Level device only has 1/4″ outputs, you will need to use balanced 1/4″ to male XLR cables. You can’t use the 1/4″ input jacks on your Apogee device, as they are only used for guitars/basses/keyboards.
Launch the Apogee Control app (Apogee Control, Apogee Control 2, Apogee Maestro 2) from your Applications folder, go to the inputs and ‘Analog Level’ drop-down menu for whichever input you’re using to ‘+4dBu’ (for use with professional audio gear) or ‘-10dBv’ (for use with consumer audio gear).