I would be careful if removing parts of the entertainment center floor, it may add considerable strength to the design and the whole thing may crash down. Have you checked the port resonances on that vent? Sometimes you can make a pipe organ with big vents if you are not careful
How far will the sub box be from the back wall? I assume a couple inches at least to allow for the cables to plug in to the amp, but I would want at least 6" if there was a vent on the rear. It would seem that putting the driver and vent both on the front would be the easiest, but you are going to have to put an elbow in that vent to get 30". If you are still thinking of using this as an end table that would be the best solution, and you could put the vent on a side and not have to have the elbow.