[ why did you remove your posting, took some time to answer all of your questions ... ]
> Does +6db mean double the volume?
http://www.sengpielaudio.com/calculator-levelchange.htm
> I tried balanced now on di input which supports balanced, but it still has disturbances but not so much anymore.
DI input ? What are you talking about now ?
What do you want to connect to where using what type of cable ?
> I had the krk rokits rp7 attached to the fireface uc and they sounded so bad for mixing
> that I thought about unbalanced and balanced and that it may affect my speakers.
Balanced connections are more resistant against noise and run on (higher) studio levels supporting longer cables. So it's a little louder and louder sounds better to our ears (psychoacoustic), but at the same listening levels the sound is the same.
> Support from thomann said, good studio monitors cost 1200 euro a pair, far beyond what I would spend for a monitor.
> Can this really be true? Does it facilitate mixing that much?
Try it out. Go to a shop with high quality music and listen.
> Like with interfaces, they have gone incredible these days, even low-budget ones.
> I think that the fireface uc is exceptional in sound and clarity
> but principally a device from 2021 could hold up with the fireface which released in 2009 already,
> or am I totally wrong here?
> Isn´t the main selling point of the Fireface UC the exquisitely reliable drivers even for the old devices
> like fireface 400 and the superb toalmixfx mixing software with lots of capabilities?
This article gives a few examples what makes RME interfaces different.
See also the Excel for feature comparison: https://www.tonstudio-forum.de/blog/ind … B-MADIfac/
Most if not all other vendors - especially applicable consumer devices - do not have such a long product lifecycle
and are far from being able to deliver such a round package where everything has a high quality including documentation.
Often you pay quite high prices already and are still far from getting the overall quality and features of RME including a very mature digital mixer software like TM FX and TM Remote, plus add-on software like DIGIcheck, etc ...
Other consumer devices of other vendors do not even have a FPGA and use 3rd party chips for the USB/FW/... communication towards the PC where nothing can be fixed if there should be a bug in the chips implementation.
Well and the product lifecycle of RME devices is much longer, for some devices already around 20y.
P.S.: just noticed, you got already a lot of informations to this and similar topics in this thread: https://forum.rme-audio.de/viewtopic.php?id=34056. Would have been better, if you stay in this existing thread about UC .. then its better visible, what has been explained to you already to that topic.
BR Ramses - UFX III, 12Mic, XTC, ADI-2 Pro FS R BE, RayDAT, X10SRi-F, E5-1680v4, Win10Pro22H2, Cub14