I once reviewed Genesis 6.1 speakers - back in 2005. I had them on the spikes, but it was also on carpet with padding underneath.
The bass was very "woolly". I wrote Gary Koh about the sound quality. Here is a part of the review addressing that.
"Another consideration, this one to be addressed very carefully, is the surface on which one places the 6.1s. In my initial setup in the upstairs listening room, I sat the 6.1s on two area rugs with floor padding underneath. My first observation was boominess in the bass, and I spent weeks moving the speakers forward and back in an effort to eliminate this problem. Nonetheless, the 6.1s refused to fully cooperate anywhere below the lower midrange. The owner’s manual says nothing about this, and the 6.1s come with no recommendations for spikes, cones, or feet. Eventually, I removed the area rugs and set the speakers directly on the padding. The bass not only stopped booming immediately, but the whole mid-bass and low bass took on a definition and clarity that astounded me. Of course, a speaker whose specs indicate that it reaches to 16Hz should shake the room, but this was not, at first, my experience. Through a blitz of e-mails exchanged with Gary Leonard Koh, Genesis’ CEO, it came to light that the speakers, if placed on carpeting too thick or too thin, might demonstrate problems in the bass frequencies. Understatement."
This was the only speaker I have ever had that problem with, including several of the most expensive speakers of the late '80s and '90s (several of which I owned). The only real difference is that most of the "big guns" I owned were when I lived in an apartment on the West Coast (San Francisco), where the basement ceiling below my apartment was concrete ( I was on the first floor), and that house's foundations were clearly more solid than the East Coast house (the one I live in now and, grew up in, and that I moved back to around the turn of the millennium), a ranch built in 1965. (The West Coast house was an Edwardian, built around 1910, when, I suspect, building standards were higher - especially since it was built in San Francisco - 4 years after the "Great Quake"). Buildings were sturdier after that event, and even when the 1989 "Great Quake" occurred, and I lived in that house, I heard a joist or two crack, but otherwise, my apartment barely shook during the quake. I didn't even believe it was a 7.1 until later, although my then Goldmund/T3F tonearm picked up the vibrations before I felt them, and lifted the tonearm off the record - about 2 seconds before I felt it. (I remember looking at the turntable, thinking, "NOW what?!!") - and then the quake arrived, moving from the back of the house to the front. The turntable was in the room "ahead" of me, closer to the back of the house, accounting for it feeling the quake before I did!
So, floor foundation - as you already know - affects the sound. Location, location, location!