Hello,
The woofers, port dimensions, port depth, and enclosure size are all designed as part of the M60Ti's bass-reflex system. Plugging or obstructing one or more of the ports entirely changes the parameters of that system with wildly unpredictable results (usually negative).
Any boominess you might experience with the M60ti, or any other speaker for that matter, will happen because of "adjacent boundary effects" (nearby walls, floor) and standing waves dictated by your room's dimensions. All rooms produce these effects to a greater or lesser degree.
To lessen these effects, try repositioning the M60s so the distances between the floor, the two nearest walls and the M60's woofers are unequal and the most dissimilar.
Regards,
