The MC200 is a Linux PC (busybox) with a Meridian designed audio board. The MC200 acts as a store (NAS box), core and audio endpoint.

Busybox and NAS have high reliance on internal disc drive.
Core takes data from NAS, converts to PCM (at same bitrate etc as source material) and transmits over network (internal in case if MC200) to audio endpoints.

As you can see, it's a distributed audio system all running on same PC in a relatively noisy environment with different needs from the power supply.

Adding MS600 moves the audio component out of the noisy environment into a dedicated audiophile quality environment. In analogue setup, very good on board DACs provide quality equal to many highly regarded dedicated DACs. On a digital environment, better control of jitter and decoupling from PC environment assist in making a cleaner digital output than the MC200 alone provides, making it easier for downstream systems to do a good job of decoding. The mileage of this depends on how tolerant your downstream systems are of non perfect digital feeds. Do a search for 'jitter' to learn more.

It is possible that the DAC on the MS600 exceeds your own dedicated DAC making that redundant in the Sooloos system.

Meridian owner since 1992
Prime & PSU, Focal Elear headphones, Roon (ROCK 8Gi5 NUC), Explorer 1 & 2, F80. 200/203, various Sonos (via Roon).