Just in case you still have all these devices available
and you are interested, at all, I would still ... let's say ... encourage you to check and compare the subwoofer settings (default or not) of them all.
That might sound a bit like I'm either anal about subwoofers or a WiiM fanboy (in reality I always was kind of a NAD fanboy) trying to defend his beloved product. But that's not all true.

I do like my WiiM Amp, but for me it's a bedroom system only where it does more than well enough. I am a declared convert (really pro now) when it comes to subwoofers for two channel audio (not in my bedroom system btw., at least not yet). On top of that I'm sure that small things can have a big influence when it comes to subjective perception.
Let me get straight to the point: Your test setup looks rather simple, indeed. Swap out one amp for the other, keep everything else unchanged. But if (and only if, of course) the crossover frequency, the filter slope, the relative level or even the phase (continuous phase aka. delay, or maybe even 180° polarity) are different between those amps, the comparison
might just not be apples to apples. Even a relatively small difference in a certain frequency range might just lead to a very different listening expression. Or maybe not. You can check and validate, if you like.
In the end, what sounds better to you simply
is better for you. Period. And the price difference from the WiiM Amp to the Edge to the C700 is such (thanks to
@mikessi ) that better sound from the latter doesn't really make the WiiM any worse for its price point. Sometimes miracles happen, mostly not. But just
really make sure you're maxing out
each of the devices.
As a final note, I confess that I do strictly believe in the benefits of room correction, if done right (now). Unfortunately, the C700 (in contrast to other NAD BlueOS devices) and the Powersound both skip that. WiiM will come up with its prorpietary RC "real soon now".

I cannot say how it will turn out. I'm not on the beta programme, unlike some others. I'm not even sure it will convice me the same way that Lyngdorf's RoomPerfect has. But it's a potential future bonus for the WiiM Amp.