I'm not certain that resonance would be a problem with a floating floor, especially using a dense foam underlayment. But my system has evolved and been tuned while being spiked to the slab, so I don't want some big sonic curveball.Indeed. I can't say from experience whether you'd have resonance issues or not (though like you I'm inclined to think not). But you're accustomed to speakers which are firmly fixed to an immobile floor. Any compromise there will let the speaker cabinets move, and that softens dynamics and smears each waveform launch.
I've heard this in many setups including early versions of my own. The slightest elasticity beneath a speaker is audible. A floating floor might be preferable in the listening area, but not beneath the speakers.