In my opinion it's best to not raise the sub unless you have to. The low frequencies of the sub often resonate with the level that they are placed on. Meaning most subs that are placed on the floor react together, giving you that common "rumble" feeling that sub users seeks. Raising the sub can also cause frequency nodes (or Standing Wave forms). This can Live'n parts of the room you may not want resonating, such as, the corners of the room and shallow, less dense areas in your rooms walls. If you must raise it try to physically keep the sub grounded as much as possible to replicate the same sound and feeling. Let me know if that helps, I'd be glad to help give more feedback.
Lead AV Tech, AA Rental