As other people are mentioning, carpet would do little for your sub, but you might want to consider it to help your LCR speakers by placing it in the path of the first reflection point between you and your speakers.
I just added an Auralex SubDude HD to my SVS PC-13 Ultra DSP subwoofer. It took me a while to figure out whether I could hear what the manufacturer's graphs suggested I would hear. Vibration transmission/creation into the floor is reduced, so I can no longer feel as much bass with my feet on my wood laminate floor. What I was very disappointed with before, especially considering how big the sub is, was the performance on bass sweeps in movies and music (e.g. demonstration of the Jericho in "Iron Man", 2Raumwohnung). It just did not grab me. Now those are a lot cleaner (less muddy) and I love how it sounds.