I have tested the whole range, including BL 4000 and BL3 as center speaker. both 7-1 and 7-4 outperform the rest by far....furthermore it is quite difficult to get a single BL3.
I would strongly recommend to stick with the 7-1...
cheers
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BV4-50, Beosystem 3, Beolab 5, Beolab 3, BV3-32, BV1, BS9000, Beolab 4, Beolab 2000, Beo4 Cinema