I have a BS3200 conencted to my Avant and the speakers on the Avant.
How I fudge it:
1 - I'll use my STB by pressing SAT which turns on the Avant
2 - Use the onscreen program guide to find the radio or music channel I want to listen too
2 - I then turn off the avant (normal beo4 red button)
3- list on Beo4 until I get AV and then press SAT
This turns on the BS3200 - sound through the speakers (connected to the Avant) and the Avant picture off
Strange and long winded but works for me