Ron was almost right 😉
I assume you are using mt-daapd from the offical version (0.2.4). This will NOT work.
Soundbridge is a product of Roku. Fine, works with both iTunes and RSP compatible servers.
Pinnacle licensed it for distribution (mostly in Europe I assume). Same system, different firmware. Does NOT work with iTunes.
mt-daapd version 0.2.4 does ONLY iTunes compatible streams. So forget it.
Or, switch to the latest nightly build 1348 atm, which will work for you.