Downfiring subs on carpet?

I have a down firing REL sub on order and was wondering as I'm looking 
at my overall setup. I have a short pile rug under my entire setup. Should 
I leave everything as is or would you recommend a hard surface i.e. a 
large ceramic tile between the sub and rug?

Thanks for any guidance.
I've got a slab of marble under mine and it seems to work Ok.
My pair of Zu Undertones sat on carpet for years. A couple of months ago I placed wooden cutting boards underneath them, and to my ears/brain, I think the sounds is more defined. 
I have down firing subs on a wood floor with a rug in front of the speakers, and a thick rug mat. I also found that isolating the sub and adding a second sub made big improvements. In my limited experience, I haven't found a difference in the front firing sub I've tried. 

Rather than a subdude pad, I used feet. SVS makes sub feet, but I went with platinum silicone isolation feet from Hudson Hi-Fi:

Rug mat:
It is very important that the sub can not move or vibrate. It should be coupled to the floor as best as possible. Spikes are the best whether the sub is on carpet or not. Right into the floor no pads. 
Coupling the subs to the floor would probably tend to increase the mechanical feedback problem, don’t you think? That’s why they invented isolation for subs and speakers in the first place. Hel-loo!