Do Subwoofer Sitting on a Concrete Foundation perform worse?

[Flaesh]

Notice how the bass is dropping off like a rock for both subwoofers? I would wager that that floor is acting like a giant bass trap

I don't know. I don't know the anechoic frequency response of these subwoofers, and the details of the room are unknown.
My subwoofers are currently leaning against the walls standing on floor. The room has a wooden ceiling and floor and solid walls. There are some graphs in my thread.
True audiophiles often suffer simultaneously from both room modes and bass sucring caused by the building's structure).

 

[NTK]

When you simulate a listening room, which I have seen you do many times, your walls have perfect reflectivity of 1.0. Would it be possible to change the reflectivity coefficient of one surface, e.g. the floor, to something like 0.5 and simulate the result?

Oh, I forgot. You can run room sims of rectangular rooms with REW. I played with it a little bit. With the right conditions (listening/speaker positions, room dimensions, etc.), you can see the effect on some modes.

Here all walls have absorption coefficients of 0.10. You can see there is a peak at ~63 Hz at ~93 dB (first axial height mode).

Change the floor absorption coefficient to 0.5. The peak went down by ~ 10 dB.