First of all thanks to everyone for weighing in, based on all the feedback I have done some more testing. Unfortunately I don't have mics or any good equipment to do a proper measuring however. I have found something interesting. This time I tested purely using the the B&W 707 speakers as I still find these to be more spacious sounding regardless of the source.
In contrast to my previous testing I didn't play back the track directly through the Sonos app but used Roon and used airplay protocol for playing back the same track to both the Sonos amp and Marantz (cutting out the Sonos port). To my surprise I started to find both amp's starting to sound much more similar with perhaps the Sonos still having a bit more full bodied sound and perhaps a bit more forward sounding but overall much more evenly matched. I later also tried airplaying to the Sonos port and found the sound to be less contrasting then before but perhaps still sounding a bit better when played directly through airplay to the Marantz. I'm not sure what made the difference perhaps the way Sonos handles tracks when played directly from source rather then receiving an airplay stream are a bit different either way I'm more convinced that the amp's are fine though I did notice I had to set the volume level much higher on the Marantz in Roon to get a similar sound that I got from the Sonos amp with a lower volume setting but that only makes sense so I might still go for a tiny class D amp as suggested to if it makes any further difference (is the difference in Ohm load audible?).
Lastly a bit off topic but am I crazy for liking the B&W much more? I've seen the KEF LS50 Meta almost universally praised as great speakers whereas B&W speakers gather quite a lot of criticism and while I have to admit that when I play back vinyl there is definitely a hissing issue (sibilance) they do well with most others playback situations (especially clarity with vocal and dialogue is amazing). If the sound wasn't that bright I'd say there's no contest here, but I think perhaps that may be fixed with a phono pre amp or tone controls?
also last edit: yes it's indeed the same amp
https://www.audiosciencereview.com/...ds/review-and-measurements-of-sonos-amp.6957/