The combined cone area n x Sd is the desirable opening for an IB manifold.
This is achieved automatically in an array since the front and back of the drivers "see" the full area of the cones without any constriction or obstruction.
Anything less than n x Sd must reduce sound quality by causing some pressurisation or constriction in front of the drivers.
It follows that if a bottom plate (or the floor) are too close to a downfiring driver then the skirt area will be less than Sd and some pressurisation will occur.
This should be very easy to prove in practice since one merely changes leg length and retests.
Physically lifting a sub from the floor is another matter unless the bottom plate is removed. Otherwise it would only affect the coupling of the entire subwoofer to the floor boundary.
I find it interesting that SVS do not downfire their large and efficient ports. While some compact subwoofers do. This can't be a simple case of cramming long ports into the box. Almost any port configuration could be achieved by using elbow joints. Whether this is desirable is another matter.
I find it hard to believe that the SVS ports have been arranged purely as a cosmetic feature. The proximity to the floor of downfiring ports must be disadvantageous from a performance point of view. Could it be a simple pant's leg flapping issue?
