Small observation on down firing subs

[got rice]

Ever since I bought my klipsch SUB-12 I have read many threads and noticed that people who have hard wood floors will place a piece of carpet under it to remove some of the boominess and keep it from dancing across the floor.

However the ones with carpet add a MDF to replicate the sound of hardwood floors, tile, marble etc.. this apparently helps with the "muffled" response the sub produces on carpet.

Why is this? I have carpet and want to know should I add a MDF underneath or will I get the boominess of hardwood? Which I would like to avoid.