Bought the WiiM Mini to be able to stream Tidal Connect to a now old B&W Zeppelin Wireless. Glad I also got the SMSL SU-1. Today I plugged my projector's line out directly to the WiiM Mini's line in (and then via optical through the SU-1 to line in at the Zeppelin).
Noise floor's height was unbelievable. Ol' Led was purring louder than the cat when it spots the gecko.
I've never had this problem connecting my projector (an XGIMI H5 whose DAC I have always--and maybe wrongly--understimated) directly to the Zeppelin. True that there is ADC^2 + DAC^4 when the sound goes Projector > Mini > SU-1 > Zeppelin. But take the Mini out of the equation and any problem is gone.
In short: I always take these measurements with a grain of salt, but I'm astounded as to how attrocious the WiiM Mini's ADC probably is. Anyone who needs to hook up other gear to the setup should get the Pro Plus instead. I'm now needing to get a Toslink to Coaxial converter to connect the projector directly to the SU-1.
My consolation is that when I am streaming music from the Mini through the SU-1 to the Zeppelin... Wow. It's just heavenly. I can't imagine the Pro Plus being better, though it would probably be cheaper, all things considered (and more aesthetically pleasing).
Edit: the problem above has been solved. It was all ground loop. Still apparently introduced by the Mini, but solvable. Thanks,
@antcollinet ! What I cannot solve is my Zeppelin's intolerance for inputs approaching 2Vrms. Maybe it's its ADC that is the bad one, after all (sounds very fine otherwise, though).