Hi ab030,
I really do not advocate doing the phantom center-channel thing--it's extremely limiting, first because you cannot adjust the volume of the dialog during movies, and second, because it only works if you sit exactly in the sweet spot between the left and right main speakers.
If you sit anywhere off-axis towards the left or right front speakers in the phantom mode, the dialog will move away from the TV screen to the nearest speaker, ruining the illusion.
So unless you're severely constrained budget-wise, I'd suggest you go with a single M2 or M22 and, if you want, put it on its side. Yes, it will affect dispersion somewhat but you may not even notice it. In my tests using a single M2 or M22 as a center on its side, I could not easily distinguish differences using program material (movies or CDs). It's certainly audible using a pink-noise test signal, but the latter is extremely critical and there are lots of anomalies audible using test signals that are not audible with programming.
Both the M2 and M22 are ported, so you'll want to leave a few inches of space behind the speaker for venting, and keep the front edge of the speaker protruding over the lip of the shelf so you don't get tonal colorations from the adjacent furniture.
Regards,
Alan
