I own the WiiM Pro, however I connect it optically into external DAC, Yamaha CD-S2100. I also tried both optical and coaxial digital outputs on the WiiM and optical sounded better.
The PSU that comes with the Wiim Pro & Plus version is poor, I power mine from a USB port on my HTPC, that has a Seasonic Prime PSU in it.
Hans Beekhuyzen covered the above, however I had already worked out myself how poor the WiiM's included PSU was.
The WiiM streamers deliver great value for money and are upgraded by many using an external DAC. But what if you replace the original power supply with a bet...
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The other thing I've noticed on the WiiM is it sounds better on ethernet, even WiiM say WiFi can cause interference, I just disable WiFi on the WiiM. I'm using galvanically isolated ethernet into mine, together with better DC power, then using quality external DAC sound quality from the WiiM can be very high.