Hi @TimPuk,
actually nothing... 
I adjust the levels via the inbuild amp function. I also did a bit of equalizing from within Sonic Pi (cut lower frequencies in order to avoid getting a muddy sound).
This is probably almost a philosophical question what you consider should be part of your live coding and what should be part of the post processing with external software/tools. The main reason for my use of Carla was/is, that while using Sonic Pi and the inbuild effects you can easily go over the performance limits of your hardware. So it might be a good idea e. g. to use an external reverb because the internal effects (might or might not) eat up a lot of CPU. There are more reasons for applying external effects (such as the variety of choice and/or the quality).
Geat! Thanks a lot. I am still working my way through the basics but this is precious knowledge for later.
That's also nice to know (though I am kind of more into OSC as a proud owner of a Monome grid )
I am very pleased of what Tidal is capable of!