I would venture a guess that it is a combination of room acoustics and speaker placement/alignment. But without pics of your room and your setup, it's a hard thing to diagnose.
Are you sure your listening seat is exactly halfway from both speakers, and are they both the same distance from your back wall? Are both speakers toed-in the same (use the "flashlight method" that others have recommended here--it works WONDERS, and you dont have to use a tape measure and a protractor to make it work!).
Are the speakers leaning at an angle that gives you optimal soud propagation to your rears at listening position? You may need to tilt them a little forward if your seat is low.
Is the back wall acoustically treated? If the back wall is really "dead" (like with a heavy curtain, or acoustical panels, or even rough stucco) it can DRAMATICALLY effect the depth of the soundstage by absorbing those primary back-side reflections.
Also, are your speakers the same distance from the side walls? Controlling primary side-wall reflections and wave reinforment are pretty critical to getting ANY speakers to sound great, but this issue (perhaps more than any other acoustic issue) are pretty much paramount to getting your ML's really "dialed in".
Is the floor carpeted? Thick carpet can really kill the "air" of MLs.
Do you have a large glass-top coffe table in front of the listening position? Such tables can create some really aggregious primary reflections that are REALLY hard to tame.
Is there a large equipment rack or TV between your speakers? That will kill your soundstage "openness" faster than just about anything. Maybe try moving the speakers out into the room a little.
If all else fails, go to EBAY or Audiogon and buy an old Carver C-9 Sonic Hologram Generator, or a Sunfire Preamp with Sonic Holography, and plug it into your tape loop. That will give you a soundtage that is psychadelically three-dimensional. I promise...
But before you try signal processing, and adding weirdo soundfield-proccessing gear to your rig, I'd strongly suggest studying your room's acoustics, and trying to figure out the problem. It does take some time and a fair amount of special knowledge to get MLs to sound really good in some rooms. There are some other folks on this forum who are WAY more expert than me at this sort of thing, and I'm sure they might have much more to offer than I have...
Post pics of your rig on the "Members Systems" thread. That would help a LOT for us to help you sort this out.
Hope this helps.
--Richard
