Wow... three "Midways" out of the few responses so far!
It wasn't buttkickers that were used... I remember seeing a row of subs, each the size of refrigerators, along the front wall under the screen.... which is why I think they could reproduce it in a special edition as long as a person had an adequate sub at home!
Since I worked at the theater - we were in there in the light the Subs were the size of fridges and they were spaced about every 5 rows. I watched the movie maybe 10 times.....