Bass is clearer and more impactful when I stand vs. when I sit

If you need to move your speakers use furniture moving pads appropriate for your floor that allows you to slide them more easily.  After you find the best spot then you can obsess about spikes and such.

+1jc

big changes happen to your frequency response just by moving your speakers.

changes imaging, soundstage and frequency response big time

I'm surprised no one mentioned room treatments specifically? It sounds like a null like others have said and I completely agree with experimenting with seat and speaker position. Just adding that room treatment for bass control might be helpful as well. Depending on how far the front baffle of your speaker is from the front wall certain frequencies will be reflected back to you when the frequency range goes into modal territory. This is SBIR. If your speaker is further from the wall this will become more difficult to treat as the reflected waves are lower frequency. Maybe you could try moving the speaker closer to the wall. Otherwise you may want to get bass traps for the front wall or 3 to 4 inch thick panels. You could also treat corners with bass traps but those are large and get ugly if your space is small. 

I will give you an animation to try to simplify what these gentlemen are trying to explain to you. Room treatment is a bit expensive. But if money is no problem then go for it. It does work.
https://www.acs.psu.edu/drussell/Demos/superposition/superposition.html Two sine waves travelling in opposite directions create a standing wave This is a good example that isn't too hard to understand.
These are the finest. Read thoroughly and check the animations.https://www.acousticsciences.com/high-end-audio/bass-traps
And a mechanical solution that is not as expensive, but is a good bang for the buck is to minimize the resonance by decoupling. Again not perfect but good for less money.
https://isoacoustics.com/Good luck and don't get too aggravated, rooms are the biggest challenge to tame. The information is available on line, and there is many DIY projects that can be found on YouTube that work, and are cheaper.

@quanghuy147 , Do you have carpet on the floor? Wall to wall with a pad underneath?  You have a vertical node problem not a horizontal one.
I have noticed that media rooms with wall to wall carpeting have less of a problem with this. The other solution is to get speakers that do not radiate up and down, vertical line source speakers.