Hi
Are you using a Denon receiver (3801/2)??
If you are, you have the option of having a 2nd set of surround speakers (surround B). So you could get 2 pairs of bipoles, one pair as Surround A, the other pair as rear centres and keep your 771´s as Surround B (for multichannel music)
Or just get 1 pair of bipoles and use them at the sides keeping the 771´s as rear centres. Personally, I´d go this route. This would work well if you have another type of amp/receiver that has the rear centre channel/s.
Playing around with the different combos is the best help I can suggest as different systems sound different to each person!!
hope this helps
cheers