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
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