When Bill Pugh of Sumiko was installing a Stadium III in my room, he recommended that I not use the spikes even though it was on carpet. For what it is worth, your mileage may vary.
I just purchased the REL Q150E and so far so good. The owner's manual specifically recommended NOT to use the spikes with REL subs. They stated that it decoupled the sub from the floor and the design was such that each boundary, side walls, floors, etc. are used to reinforce the sub bass.
I am using the standard nylon feet on carpeting for now but I have always used spike for my other subs.
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