tuning the box from 28hz to 33hz will make a difference on how it sounds, if it will be a desirable one that's a different story. changing the tuning frequency will affect the gain level of some frequencies of the frequency responce, while changing the box size will change how wide the frequency responce is. "dont quote me on that", so they go together hand in hand, to tune the box just right,
unless you know what the outcome of the new tuning freq will be, don't do it.
so with that said, if you can get the current external dimensions of the box and the driver T/S, then get the box redesigned to sound close to what you want.
some subs have such a grate frequency responce that can cross into the mid-bass freqs and compliment the sound system better than a mid-range going in to the mid-bass freq responce will.