I'm afraid to look and see....
Actually my forward firing sealed M&K sub needs some repair work and has been taken out of active duty for now.
No doubt subs, especially one that fires downward in any shape or form, will interact strongly with the floor, especially elevated wood or plywood construction floors and placing same on various platforms that help to attenuate that interaction when needed helps if other available controls or level adjustments prove insufficient alone, which may well be the case.
Same sub say on ground level carpet covered concrete foundation may need nothing special underneath to help tame it.
So what specifically is under the sub in addition to the floor may not matter as much case by case as having the right thing under there in each unique case to help tame interactions with the floor if needed. I would expect the need in general to be less or nil in general on a concrete foundation/slab. There may well be other acoustic artifacts of teh room beyond just the floor though that come into play. SO you can apply some general rules, but hard to say for sure what is best with out hearing.
I have speakers with bottom ports that I place on more rigid tile platforms to help tame floor interactions when used on my second level with typical plywood/carpet floors, yet same speakers in my basement level on concrete foundation with thin carpet need nothing more underneath them, though I do apply other tweaks as needed there in order to get bass levels just right.
