Hum..tough one.to match the tone of the rest of the speakers, maybe the MA's br1, or Cambridge audio s30.or the radius( albeit more expensive) or just try to get some used kef 2005.3 ...tricky that one.
You can buy the KEF 'eggs' in pairs, so i'd add another couple of 2005.3 speakers. You could even add their little brothers - the 1005.2 speakers - if you're looking for a cheaper/even more compact solution.
I have the same kit as you and have been thinking about adding 7.1 but the cabling runs now are impossible so I have given some thought to the KEF wireless rears not cheap at £400 ish but the guy in Superfi said they are good and it will work a treat. Meantime still thinking and I will have to get it sanctioned by the in house accountant
You can get a few different types of KEF speaker from Superfi - the 2005.3 model speaker is here as a single speaker purchase or here to get a pair, but they also have the 1005.2 model as mentioned by Clare here and here.