Decouple, because the floor boards are a big, flexible, poorly braced structure which will radiate sound.
If you have nice speakers where even the sub-woofer enclosures are braced every 7-8" to keep panel resonances out of the pass-band, why would you put them on top of a wooden structure that isn't even braced every 10'?
If the bass drivers aren't mounted in a bipole configuration you'll get more rocking of the speakers when they're not anchored to the floor although the speakers are so massive compared to the drivers' moving components (even a heavy sub woofer isn't half a pound, when a light weight speaker cabinet is 50+ pounds) that the lost driver motion is inconsequential.
If you have nice speakers where even the sub-woofer enclosures are braced every 7-8" to keep panel resonances out of the pass-band, why would you put them on top of a wooden structure that isn't even braced every 10'?
If the bass drivers aren't mounted in a bipole configuration you'll get more rocking of the speakers when they're not anchored to the floor although the speakers are so massive compared to the drivers' moving components (even a heavy sub woofer isn't half a pound, when a light weight speaker cabinet is 50+ pounds) that the lost driver motion is inconsequential.