Hi,
The size of your room is sufficiently large that no matter which type of speaker you choose, there's going to be lots of reflected sound happening, so it's unlikely the sound would be "confined to the listening area."
The M3ti is not a good tonal match for the M60ti. The QS8s would be a much better timbral match and would deliver better envelopment, particularly since you don't want to mount them in the recommended side positions. Actually, why don't you wall-mount them at the sides? The QS8s are supplied with wall-mount brackets. You'd use the setup menu on the receiver to dial in the necessary delay for the surrounds at a 15-foot distance.
Regards,
