Any difference you are hearing isn't coming from the deice, but from uncontrolled listening.
If you are using analogue outputs from the Wiim, it could be due to not accurately matching the output levels - typically (though not always) a slightly higher level will sound better.
If you are using digital out from both devices, then there is no audible difference possible - in this case the difference you hear can only be due to your (very human) auditory system's perceptive bias. If you were to compare blind (so you don't know which is playing) then you would not be able to hear a difference.
(EDIT - all assuming you have not applied any DSP (eg filters) in either of the devicI decide to send it back.
It doesnt matter. If they transfer the data - and it reaches the destination without being changed - then it must sound identical.
If it is changed (data errors) then you won't get subtle changes to sound quality - you'll get pops, clicks, dropouts.
Makes sense. I’ll do more evaluating before I decide to send the Wiim back. I’m not using any EQ and it’s the same USB cable and network cable into my McIntosh C2800 DAC input. Only thing I can think of is the Wiim may be “noisier” due to the screen and maybe having WiFi not being disabled. I just hear more detail with the Zen.