See [Official] SW Ver 5.9 (Rose OS 5.9.07) Software Release - #275 by Stefan ← he solved the poor audio problem.
If you still have the problem: Just an idea. Contact your retailer and ask for a Zoom meeting. That way, he can walk you thru various tests & checks. The retailer can see how the device is working and reacting. With Zoom, you may succeed.
BTW, you said than you do downsampling/upscaling. Turn all the fancy stuff off. Then test will the most basic “all default” settings - play NAS first (not the internal disk) ← if you don’t have NAS, connect a mobile disk on one of the RS130 USB connection. Use your computer to format the test disk to say NTFS format. Transfer only a few audio files (either stereo mp3 or flac files) on the test disk. Test these files. If it works, from your computer, reformat the disk to exFAT. Test again. If it works. Format the disk from RS130. Continue the process, one step at a time till you find the culprit.
But first do the Zoom meeting with your retailer. Your retailer may decide to replace the RS130. Problem solved!
Note: If your retailer is simply a sales person, ask to have the Zoom meeting with a technical person who has detail RS130 knowledge.