Having a SAN media server does not automatically require SSO. You only need SSO if the SAN media server is sharing drives. SSO enables the drive sharing. A SAN media server with dedicated drive(s) does not require SSO.
If you are sharing the drive, you will need SSO license. If the media server is backing only itself, then you need SAN Media Server license. One thing you have to be very careful about SAN mediaserver license is that it cannot backup other clients. If you wnat to backup multiple clients, you need to go for standard media server license.