Another thing, some speakers have down-firing ports which is the same idea. A long time back I went from B&W 804s to 802s. The 804s had front ports and the 802s had bottom ports. The measured in-room response of the 802s was dramatically better in the deep bass. It was very close to spec whereas the 804s had huge variances.
I noticed Revel started doing down-firing ports on the ultima 2 line. It has its advantages.