There is so little sound material for that 6th channel, that I suspect you could use just about any speaker on the market.
But if you are serios in trying to match the speakers...
The specs of the speakers are almost meaningless for this search, you are more interested in trying to match the sound quality/emphasis of your existing speakers in the voice and lower treble range (basically 100-200Hz to 3k to as high as 8k or 12k Hz depending upon whom you ask).
Look for a speaker with similar driver types (cone, dome etc) and material (paper, fiber, metal etc) for midrange and tweeter.
My suggestion would be for you to check the close out models at CC & BB for a good deal on a small bookshelf that would basically match up with your existing speakers.