If the input signal is not high enough, then you have to check several things.
a) reference levels on analog output of the connected device and on the analog input of your recording device
b) volume settings of the connected device
Do not drive the signal hard to 0dB. Leave some headroom for peaks.
The levels of several tracks add up during mixing anyway and you need headroom for mastering and mastering tools.
After the A/D conversion on your RME recording interface the music signal is completely digitized, there is no waveform anymore, only "zeros and ones" (binary format).
There is nothing like a "analog ADAT preamp".
ADAT is simply a digital port to transfer digital data in either ADAT or SPDIF format.
The manual gives you an overview about the supported reference levels. It depends on the device whether you can set them in the driver settings or in TM FX, click there to the wrench symbol of analog inputs (and outputs).
Some devices have no consumer level anymore, but there you can usually add some digital gain, which results into compareable sensitivity of consumer level, so that not so loud signals come closer to 0dB.
By choosing correct reference levels you will have optimum SNR and Dynamic.
But you really have not to push the levels up to 0dB, it's bad to have clipping ... this is no analog tape deck anymore ...
BR Ramses - UFX III, 12Mic, XTC, ADI-2 Pro FS R BE, RayDAT, X10SRi-F, E5-1680v4, Win10Pro22H2, Cub14