sounds like that Livebox does something to the bonjour messaging done by Firefly. (multicast discovery). I mean.. if it shows up with the netgear.. and when using the Livebox (what brand is that?) only firlefly wont work. The other mediaservers you mention use multicast, atleast windows media connect.
wmc does use multicast. But it might be that the livebox uses a simple multicast proxy or a upnp proxy rather than a general-purpose igmp daemon, and it doesnt’ understand anything else.
But if the linksys works, why not use it? Just don’t connect the ethernet side. Plug the “inside” side of it into your network, and use the wireless offered by it for the soundbridge, but dont’ connect anything into the “wan” side — use it simply as an access point that works with multicast. 🙂