Kristian, good question! I find the M24 a bit richer tonally, as might be expected a more "tube-like" sound considering it's a tube DAC! That is one reason I keep using it, to get the tube element in the digital chain.
Both are superb results; the Cambridge internal DAC is more like the classic SS sound, and the M24 puts a tube flavor to it.
I often send the upsampled digital output of the Azur into the M24. Very nice combo. Sonos is usually run through the M24.
For Redbook playback I teeter between the "Direct" output of the Azur and its "Upsampled" output; the former is more clean, but the latter is a touch more nuanced.
Tube lovers would likely enjoy the Monarcy; SS lovers will gravitate toward the Azur 840C's internal DAC.
Possibly a good "test" for those considering purchase:
If you think that tube sound in the bass is "tubby" and the entire presentation unrefined, then you will most likely enjoy the taut sounding treatment by the Azur's internal DAC.
If on the other hand you feel that SS in the bass is often clinical or thin, and the entire presentation lacks body or fullness, you'd most likely prefer the M24 pre/DAC's sound.
These characteristics hold true for Digital sources like Sonos.