You have me confused. You mentioned Spotify, but that is a service which fetches its content from the Internet, and anything that gets stored locally will be protected by DRM (Digital Rights Management - ie encryption).
If what you want to do is share content that you have already stored on the Mac (standard mp3s or whatever), you need a means to make it accessible to the Humax - and than means installing a DLNA server eg Twonky. With the Mac appropriately configured, it will show up on the Humax via Media >> Storage (blue) >> Network.
Simply duplicating the audio feed from the Mac onto the Humax is not an option.