A few questions and thoughts. Can you confirm that you are using the Arcam AVR with a pair of Spendor speakers, no center speaker, surround speaker or subwoofer? Can you also confirm that the Arcam AVR is set to 2.0 or stereo output with full range speakers? Can you confirm that the Arcam does not support Tidal directly?
As a starting point, you should be able to compare the sound quality of the WiiM Ultra DAC to the sound quality of the Arcam AVR DAC.
To use the DAC in the WiiM Ultra, connect your WiiM to your Arcam using the WiiM's Analog Output and set the Arcam to Stereo Direct mode. From the manual:
Stereo Direct
To listen to a pure analogue stereo input, press the
DIRECT button. The Stereo Direct mode automatically
bypasses all processing and any surround functions. In
direct mode, digital processing is shut down to improve
the sound quality and reduces digital noise with the
Receiver to an absolute minimum.
When connected in this manner, you can select a number of different WiiM DAC filters to see if you notice any difference in sound quality.
To use the DAC in the Arcam, connect the WiiM to the Arcan using a digital optical cable or digital coaxial cable. When you do this you should confirm that all signal processing is turned off in the Arcam.
When performing comparisons it is critical that the output between the two setup are level matched as experiments using a sound meter. Otherwise you are likely to prefer the setup with the louder output.
I would then run the DIRAC room correction software that is in the Arcam AVR and repeat the WiiM DAC, Arcam DAC test.
YMMV.