UPDATE
Whilst waiting for my replacement NSLU2 to arrive (don’t ask), I managed to exchange my Belkin Modem/Router for a Netgear DG834G.
Tested the Netgear with my PC’s Firefly server, and both…
WinXP Firefly -> WIRED -> Netgear -> WIRELESS -> Soundbridge
WinXP Firefly -> WIRELESS -> Netgear -> WIRELESS -> Soundbridge
…work perfectly! 😀 I turned on the Netgear’s only ‘obvious’ Multicast setting I could find (LAN IP Setup > RIP Version > RIP-2M), and everything seems to work fine. Not even sure if that was necessary! Router version is V4, and Firmware is V5.01.01 BTW.
So things are look quite encouraging. I’ll be sure to report back if/when I get a new slug back!