Downfiring subs on carpet?

Mapman is correct. 100Hz has a wavelength of 10 feet. The only way the carpet would effect the sound is by lowering the resonant frequency of the floor by adding weight but no stiffness. The trick with the floor is to get it's resonant frequency outside the range of the subwoofer. Concrete is the ultimate. You can get wood floors stiff enough by using larger laminated joists and doubling up on the plywood. Use construction adhesive on everything. What you put the woofer on makes absolutely no difference. My woofers are on concrete. If I play a 20 Hz test tone the whole house becomes a symphony of rattles. Fortunately music masks the rattles. Low frequency sound is very powerful. In terms of distortion the most important factor besides the driver itself is that the enclosure is stiff and very heavy. Ideally when you put your hand on the sub while playing you should feel nothing. No vibration at all. Having said all this you can negate most of these problems with room control as long as they don't boost any given frequency more than 5 dB or so. After that you start running out of power and digital headroom.

Hello markj941:

     Please don’t take the advice of those sub purists who recommend getting rid of the floor altogether, you may find the cure is worse than the disease, depending on what’s underneath your floor.
     It’d be very interesting and informative if you follow lalitk’s suggestion of a SubDude platform and jjon_5912’s suggestion to try your REL sub with and without it and then report back.
     I have no knowledge or personal experience regarding whether down-firing subs perform better on hard surfaces versus on carpet.  However, I do have significant knowledge and personal experience regarding the usage of single and dual down-firing subs in my system and room with wall to wall carpeting.
     Based on my knowledge and experience, I do have a couple of sub usage suggestions to offer you that are not directly related to your question but I believe you’ll still find them very useful:

   For best bass results,  it’s very important to precisely position your sub in your room in relation to your designated listening seat, especially if you’re going to initially utilize only a single good quality sub.  If you don’t currently own and don’t want to  Invest in the fairly expensive measuring equipment to determine this optimum sub room location, I’ve found ‘the sub crawl method‘ works equally well and it’s free.  You can google it for details.

     Through personal experience, I’ve learned that the generally accepted guideline, that the bass from 2 subs perform and sound about twice as good as a single sub In virtually any system and room, is absolutely correct.  Bass is cumulative in a room and utilizing 2 subs increases the in-room bass power and bass dynamics capacity when the source material calls for it, imparts a sense of ease and limitless to the bass and ensures that both subs are operating well within their limits.  The bass also sounds faster, smoother, more detailed and better integrated or blended with the main speakers.     This is also budget friendly, since individuals can begin with a single good quality sub and optionally add a 2nd sub in the future.

Best wishes,
  Tim     

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:  https://www.amazon.com/Platinum-Silicone-Hemisphere-Isolation-Durometer/dp/B075KM5Y8W/ref=sr_1_3?dchild=1&m=A3DXW0NUYLHI2J&marketplaceID=ATVPDKIKX0DER&qid=1588947657&refinements=p_4%3AHudson+Hi-Fi&s=merchant-items&sr=1-3&swrs=C706340B8638275BE0014217667F93CC

Rug mat:  https://www.amazon.com/gp/product/B01HN8MC4S/ref=ppx_yo_dt_b_asin_title_o03_s00?ie=UTF8&psc=1