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try soft rubber or eva foam pads, makes sure they do distort under weight so they could dissipate energy, otherwise they’re too hard and will couple with the floor
Isn't this largely a matter that depends on how the floor is constructed in the first place, i.e. what is below the hardwood floor? Some possible underfloor heating, no air gaps, and then a concrete foundation directly on the ground (well, with isolation underneath that and so on) is hardly likely to actually pick up any resonance is it? At least none of my blind tests have revealed any results on such floors, but I am no authority on possible effects on other floor constructions and possible resonance. I can speculate, but that's not why we come here
Basically no. Spikes or Sorbothane were all the back full color page rage, but not convinced they do anything but transfer money. I guess is your floor was uneven and one foot was just off it, it might rock. FWIW, my sub is sitting on stick-on felt tabs so it does not scratch my hardwood floor.
Changing the feet is unlikely to alter the frequency response of the sub in any meaningful way. Stock rubber feet are probably fine. Unless it's an extremely cheap subwoofer, it's reasonable to assume that the manufacturer included feet that works well. If you have a very flexible floor (for instance a 2nd story wood floor in a house) and you are experiencing audible vibration from other objects in the room, changing to very soft feet (soft rubber compound of some kind) may reduce the issue.