The ATEM devices effectively have a -10DB pad built in because of how the A/D converters are set up. The way I fixed this for the last studio I built was to adjust the volume of the input in the ATEM Audio Control Panel to have the volume slider all the way up at +10dB. Make sure to monitor the audio levels in or after the ATEM to make sure things don't clip. If you send a test tone from the signal generator at -10dBFS to the input of the ATEM, that will allow you to adjust the ATEM signal level to also read -10dBfs.
The better way to do this would be to connect the AES output of the SI Expression 2 to a Teranex audio to SDI converter or Teranex HDMI to SDI converter. Using the optional front panel, or network connection, you can adjust the gain to wherever it needs to be and meter on the included confidence monitor.
Another essential piece of equipment for stream audio mixing is the TC Electronic Clarity M V2. This allows you to monitor the loudness of the audio along with a true peak meter instead of just guessing with dBFS. This will require a SDI to audio de embedder and AES TRS to BNC impedance converter. This SDI to audio converter doesn't have to be the fancy Teranex one, because the audio levels are not being changed. I recommend monitoring and physical controls on any device that adds gain to the audio in the system. Converters that do not adjust gain, or simply decrease gain do not need to have built in monitoring.