speaker selector

I have a pair of tannoy mx2 speakers and a pair of Kef Cresta speakers and am starting to explore whether or not to purchase a speaker selector. Could anyone let me know how the speaker selectors operate, what brands to look at and whether or not these selectors will prevent the loss of power I seem to experience when I play both sets of speakers through my A and B channels. I have a Marantz sr-48 receiver.

Thanks