It's amazing you can hear a difference between the different interfaces but good for you. You do however clearly not understand how it all works regarding the audio data transfer and keeps talking about the USB giving control to the DAC.
The audio data transfered on SPDIF and USB is exactly the same and the DAC have the same control no matter what interface you use. It's just different ways of transport with USB being package transfer that always need to be buffered on both sender and receiver side. While SPDIF can be buffered on receiver if a local clock is used or else just used directly without re-clocking.
And even if you use 32 bit USB, it's still just 24 bits upscaled, on WiiM.
Okay, great so Wiim decided to put a ESS ES9038 32bit DAC inside the Ultra and put the menu option of output in 32bit just so they can instead output it in 24bit and upscale it, is that what you are saying?
So, let's conclude this once and for all. I can understand upscaling to some degree as Audirvana does and it's pretty good. When you watch a 1080p film on a 4K TV, it upscales it because it has to.
You are getting stuck on fine details, if something is upscaled then wouldn't it have more resolution by virtue? Would a 1080p film upscaled to 4k just have 1080p pixels? No
The fact that the Wiim has a 32bit capable DAC Chip inside, I would imagine the upscaling to be pretty good, wouldn't you think?
Now would a 1080p film or a 24bit file upscaled sound/look better than a native 4k / 32bit you would have to say no. But, would the upscaled look better than the original and you would have to say, yes!!!!
So, if it arrives at 32bit I would imagine it would sound better than before and due to the way USB allows full control to external DAC for data the resulting sound to my ears vs £250rrp COAX which I tested it myself is smoother, this is real life test with my own money!!!
Now, can you please detail the system you have (you are under an alias anyway) and what cables you have tried, because I feel you either don't have a system revealing enough to hear the differences in cables or you have not purchased and compared things for yourself!
I look forward to your response, it's been a pleasure as always Mr HGO58!