..while the 818v3 will great upgrade the analogue input sound quality, the flac streaming from sooloos essentially is the same in the 818 as it is in AC200
That's technically correct in that both the AC200 and the 818 utilise the same FLAC source files, but incorrect as each device handles those files with quite different refinement, in addition to separate input methods as Ian noted above. The resulting sound quality from each is markedly different. And it's not just superior analogue sound quality, the digital output from the 818 will be far superior to the AC200.
Cheers
George