@elliottbnewcombjr is on the right track but I would do it by putting the speakers on dollies to move around and when you find a better location (if there is one) put them back down on the spikes.
As you discovered rooms can develop nodal behavior both horizontally and vertically. This is a function of your room, not your speakers. Absorbing bass is very difficult. Most room treatments that "absorb bass" do not work and moving a wall, ceiling or floor is not in the cards. I usually tell people to move their listening position to a place where they like the balance but you have a vertical problem. Still, moving your listening position you might find a better balance. Moving it up would be a sure cure but aesthetically challenged.
Next is digital "room control" which is really speaker control. It EQs your system to be more balance anywhere you place the measurement microphone. In your case it would boost the bass at the frequencies that are down. The problem with this is that it requires a lot of power and can stress many loudspeakers increasing distortion. The best option is subwoofers. These take the load off the main system and if you use multiple subwoofers you might be able to control, even extinguish the nodes in your room. The best processors combine digital bass management with room control and digital EQ. These made by Trinnov, DEQX, Anthem, Legacy and MiniDSP. I personally prefer the new DEQX design which also includes a 4 way digital crossover. They also make the same unit without the crossover for less adventurous audiophiles.
Next and last is to set your system up in a different room. I have background music throughout the house but I do all my serious listening in a room designed for it. The room you are in now might only be useful for back ground music.
Good luck, this can be a very frustrating problem and be careful. The industry is more than willing to sell you a bunch of junk that does not work.