Coupling the speakers to the floor just causes the building to add it's own response to the spectrum excited by the speakers. There is no advantage to transient performance by doing so. The speed of the cones renders the issue moot. What happens is some frequencies are boosted, muddying others with the net result being just boom. The best way to place speakers is to do the opposite and decouple them. Now you get the flattest performance with good bottom end definition. Directly flat on the carpet doesn't work because they're now using the floor as a sound board which is even worse. There are expensive devices/products that are designed to decouple. I just use soft 1" thick 12"X18" fiber pads used for floor polishers from Home Depot with a 1/4" rubber mat between it and the speaker. They absorb virtually everything, the floor does not vibrate at all. The large 12X18 pad takes a lot of weight spread evenly over it. A stiff board placed on top might be needed if your speakers have a small foot print. I currently have my two Velodyne subs on this set up. The difference is quite dramatic. This is at least a very cheap experiment I would encourage all to try. You may be pleasantly surprised.
This set up is also very beneficial under your tube gear.
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