For the surround speakers, in my opinion, you can get away running non-matching (to the front speakers) without really a big deal at all (with movies surround sound, not necessarily as true to with multi-channel audio). I think it is more important that the front channels (left, right and CENTER) be better matched. Based on this belief, you may want to consider adding a B&W center channel speaker (I believe it would be the HTM model that would go with these, but I am not sure on this). I would not go with a B&W sub, as I don't think they are really very good in comparison to others that you can get. For me, I much prefer REL (maybe an older Storm).
Another future consideration would be to add a good 2-channel amp and run the pre-outs from your receiver to this amp for the front two speakers (left and right) to get more enjoyment out of music (2-channel).
Another future consideration would be to add a good 2-channel amp and run the pre-outs from your receiver to this amp for the front two speakers (left and right) to get more enjoyment out of music (2-channel).