Isabel, I experimented quite a bit using test tones with my M22s. The basic idea is to have the tweeters at about ear level in your usual listening position. I found that the pattern a little above the tweeter axis was a bit better than below-axis(although I'm a little puzzled by a couple of reports of the opposite), so if anything, I'd suggest going somewhat low on the stand, something like 16-18" depending on your sitting height.
If you mean a back center surround, it isn't essential, at least at first, but your 1400 has seven amps and if you get the latest version with the DPLIIx upgrade, in the future you'd be able to have a stereo effect on a sixth and seventh back surround speaker derived by the DPLIIx processing from two, five or six channel recordings.
