Niles is probably one of the better ones. They have them with and without gain control. You should get ones with protection for low impedance if you are connecting all 6 pairs to one (or even 2) amps.
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I use a Bryston unit with three selectors. I think they make a larger one as well. No impedence matching, no volume controls. Expensive. I did some research on them about a year ago and read a couple of comments in the trade rags, and in real-world reviews that claimed no coloration using the Bryston. It works well for my purposes, which are rather simplistic. I believe Niles Audio makes all kinds of selectors which do include impedence matching, but that necessarily means putting something in the signal path. If I understand it correctly, the only time you'd need this is if you were running more than one set of speakers at the same time. I never do this so it is not an issue. If your speakers are simply for ambience I would not think much of this may be at issue at all. If they are for hardcore two-channel listening at one or more selector positions, you may want to research your options carefully.