You can only have one USB DAC connected to the Synology so there can only be one output via that method.
You can have as many DLNA clients/renderers, or players that connect to network shares on a file server, on your network as you want and the Synology will serve mutiple independent streams to them. There may be a limit depending on processor power etc. Each player will have its own red/white or optical output that you can feed into amplifiers or audio distribution systems. But DLNA devices require DLNA controllers, the actual method of browsing and picking files and sending them to a device.
SqueezeCentre is another built-in server in so if you have a Squeezebox system then you can use the Synology to serve files to that rather than using a PC.
Sonos can connect to UPnP servers but it only knows about Windows Media Player! So it won't be able to receive catalogued files from a Synology, except by using the Synology as a simple file server to hold the physical files.
I have no idea how Russound works or how it achieves connectivity!
Remember that a Synology NAS is simply a small computer and what it can do is store and share files and it also has built-in media servers, but it is limited in terms of being a media player and producing output itself, it can only send files and various streaming protocols (DLNA, Squeeze, network shares) to other devices