Watch out if you go too low with your stream as the Mp3PRO plugin gives an anoying whine that starts of low and builds to a headache inducing screech when connected to a low stream PRO output. It seems fine at higher rates aroung 40k+ but defeats the object of being a big bandwidth saver down at the low end. (it just needs some more work I think)
Ive only used the plug to listen and not to stream Mp3PRO though.
we use the Sam Encoders plugin
its a 'pay for' but it has Mp3PRO & Vorbis and can handle multiple streams to various servers at all at once too.
You could try the Oddcast Ogg vorbis plugin
from Oddstock. Its free and it allows low bit streams (down to 12k mono) and gives a high quality audio output. 20k can sound as good as 56k. Check it out.