Do speakers sound better on wood floor or concrete

Concrete, unlike wood, does not audibly resonate. Concrete, with or without rugs, does nothing to bass - rugs will not absorb bass frequencies. It might sound damped compared to wood because the wood augments the bass via resonance/vibration (even with spikes). There's a "whole world" dedicated to "room tuning" and it based on the fact that wood resonates and by doing things to the wood will control the resonance that modifies the sound. This is also somewhat separate from acoustically treating the room to (hopefully) get rid of "hot and cold" spots in the room and the reflections that cloud the intended sound when using your speakers/gear. Given the choice between concrete and a "regular" wood floor, I'd take concrete - you can always add something later modify the sound if you think it's needed.