I do this with my Rel Strata III down firing sub. I flip it over so it is up firing, put a nice thick piece of wood on top of the now upward sitting feet (I use a large 1" thick maple cutting board) and put something heavy on that. I also recommend getting it out of the corner. You will have to turn it up a bit louder but with both of these changes you will now have something that is absolutely faster sounding with better overall pitch definition.
Down firing sub tweeks
I use a down firing sub on a carpeted surface. Are there any specific tweeks available to better disperse the bass ? For example, would a product like Auralex sub dude make any improvement, or is that just to reduce the bass below the unit? ( Like in an apartment setting). Anyone use a "base" of some type, or is there is there an isolation base surface known to be better than carpet?