My bet is that you need three cables. One to connect the sub to the sub socket on your BV8, and one each to the rear speakers, from the corresponding sockets on your BV8.
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Happy owner of: BV10, 2 x Beocom 2, Beocom 6000, Beocenter 2, Beolab 3, Beolab 2000, Beo4, Beolab 11
1:st prize winner Beoworld January Price Draw.