I did think of going for 685s all round for my system originally (before I realised there wouldn't be room) but was advised by someone at Sevenoaks that this would be over the top. However, given you have the 684s as your fronts, this might work quite nicely. Is there anywhere you can demo to see if the difference is that noticeable? Rear effects are becoming increasingly crucial these days.
Of course you could get the 686s, then eventually you might look at going for a 7.1 setup and push the 686s to the surround backs and then get 685s for your rears...