How far apart have you placed the M3ti's? Try separating them by at least 6 or 7 feet or more, with your listening position at the apex of a triangle.
And do check your amp/preamp/receiver--it's easy enough to inadvertantly switch something to mono and forget to return it to stereo.
Is the disc you are listening to recorded in stereo? Record labels are quite cunning at concealing that fact, especially with vintage material recorded before the late 1960's.
The M3ti's normally deliver an excellent stereo image when they are set up properly. I'm sure the problem lies elsewhere, unless you are listening in a highly reverberant room (say, with a cathedral ceiling) a long ways back from the speakers. In such rooms, there's seldom a detectable difference between mono and stereo recordings because most of the sound reaching your ears is comprised of relections from the room surfaces. The stereo cues (time and amplitude differences between channels) have been lost in all the reverberance.
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