JRiver is capable of outputting in different formats than those programs; they all use DirectSound. So you might check how MC is configured, and look at Audio Path during playback, while connected to the Singxer. Perhaps the Singxer cannot digest what MC is outputting. Your DAC isn't showing you what MC is sending, it's showing you what the Singxer is sending.
The problem may be in the JRiver audio config for the Singxer device, or in the config of the Singxer driver, or in the config of the device itself. It is evident that the problem lies in that part of the chain.
But what are you trying to achieve by having the Singxer between your PC and the DAC? Do you think it is somehow improving things?
You have a good quality DAC, that is capable of accepting the USB data directly from MC. Letting MC talk directly to your Gustard DAC via USB will produce the best results of which your system is capable.
In fact, your exact device has been tested over at AudioScienceReview, while hooked up to a Gustard DAC. The conclusion was that the Singxer made zero improvement when connected between the PC and the Gustard DAC. Take a look:
https://www.audiosciencereview.com/forum/index.php?threads/review-and-measurements-of-singxer-su-1-usb-to-s-pdif.7095/There's also an analysis of the Singxer's performance over the I2S interface:
https://www.audiosciencereview.com/forum/index.php?threads/study-is-i%C2%B2s-interface-better-for-dacs-than-s-pdif-or-usb.7105/I understand there are various "mods" available for that DAC, each of which of course "takes it to a whole new level". Perhaps that's the avenue you're pursuing with the Singxer.
Good luck...