Its usually bad to put a sub in any cabinet. Just like it can be a problem putting a sub in a corner. Not only will it shake what's on the shelves, you will get resonance problems (you are putting it into another box). The only way around that is with an equalizer.
Can you put the sub on top of the cabinet? It may be more open up there?
Can you put the sub on top of the cabinet? It may be more open up there?