Now you eventually lost me.
I thought that streaming to the WiiM Pro Plus (using Spotify Connect) and using the WiiM Amp as a power amplifier was the scenario that didn't work?
And one more time, even if it's been said before: If you connect the line out of the WiiM Pro Plus to the line in of the WiiM Amp instead of using the WiiM Amp alone, the what you do is adding the DAC of the WiiM Pro Plus and the ADC of the WiiM Amp to the signal path.
One additional DAC. One additional ADC. The DAC of the WiiM Amp is still in use. There's no way around this as long as you are using the WiiM Amp's speaker outputs.
Your new setup:
Spotify Connect =>
[(WiiM Pro Plus) digital network streaming input => digital processing (EQ, volume, etc) => DAC => analogue line output] =>
[(WiiM Amp) analogue line input => ADC => digital processing (EQ, volume, etc) => DAC => power amp section => speaker binding posts] =>
[speaker cables] =>
[(speakers) binding posts => crossover => speaker drivers]
Your previous setup:
Spotify Connect =>
[(WiiM Amp) digital network streaming input => digital processing (EQ, volume, etc) => DAC => power amp section => speaker binding posts] =>
[speaker cables] =>
[(speakers) binding posts => crossover => speaker drivers]
There's really nothing to gain from the new setup if all you want.to do is playing Spotify in ... errr ... "better quality".
If you still just want to know what's going on (which I could understand),
@Wiimer proposed pretty much what is possible.
The most important thing is that EQ can be different per input, but
@Wiimer had that covered, too.