I think buying a used beosystem is sort of tricky unless you get one of the few screens that actually works with it. If I recall correct, panasonic worked besides B&O but not much else.
There are surround amps with regulated line out (i.e. you can control the volume, so it's not a steady line out). If your speakers don't have a line switch, there is a way to supply them with a 5Volt power so they switch on and stay on.
Personally I don't see any other acceptable way way then buyng a B&O TV. You spend a fortune on speakers, why fiddle with that and downgrade it with converter boxes. I was eyeing the AV7000 quite frequently on ebay but just feel it would be best to save up for a BV7, hoping that a smaller one comes out some time soon.
JK
BS9000, BS2300, BC2, BL2500, BL3, Bl2, BS1, BV8, BC4, A8