Material to place under Down firing Sub ?

For what it's worth, Sumiko recommends that REL subs (downfiring) be placed directly on the carpet, without spikes, when the floor is concrete. I doubt you'd find much difference with placing wood, or even a slab of granite, under your sub when the floor is concrete. As always, try it and do whatever sounds best.

You may also want to do a quick search through the archives, here is one talking specifically about REL subs. Good luck.

http://forum.audiogon.com/cgi-bin/fr.pl?cspkr&1049257223&openfrom&1&4#1

I have mine on wood over concrete floor, the sound improved a lot with spikes on...

Having used many RELs, I have to disagree with the above Sumiko recommendation. The only time it is appropriate to go without spikes is if the sub is sitting directly on a hard surface. Otherwise, you should use the spikes and adjust them so that the air gap between the cabinet and the top of the carpet is the same height as the legs. In other words, the legs should graze the top of the carpet. Thinner carpet = shorter spikes, thicker carpet = longer spikes. Putting the legs directly on carpet results in the cabinet being "suspended" by a crude spring and muddies up the bass. If the carpet is hard and thin enough to be virtually the same as a hard floor, then you can probably get by without spikes.