Besides other happens, like the singers throat, the mic, or the preamp settings:

Might be a clock problem, or you have the Digital Limiter active and your level is hot.

You need to decide which unit should be clock master.
All others need to be clock slave.

As the RME converters are quite good beeing clock slave, the Apollo / DAW can be master.

Now connect both ADI-8 ADAT in AND out to Apollo, and set ADI-8 Clock (middle section) to:

• Clock: ADAT

• On the left, switch: Limiter: Analog

The settings for Apollo can be fould here:
https://help.uaudio.com/hc/en-us/articl … ng-Primer-

Input +19 is ok.
Out + 13 is ok, but also it is irrelevant as you use digital output not analog.
Watch the input level meter in the QS, is it going into red ? Lower the preamp gain.