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

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.

Sounds like you have a bass null at the listening position.  I have the same situation, i.e., there is more bass in my room pretty much everywhere except in the listening position.  The bass is great at the sides and far back of the room.  The problem is that this is a family room(don't have a dedicated listening room) and I can't really arrange the furniture much differently. 

Null, node; these phenomena are caused by standing waves in the room. A standing wave in this case is where a bass note is so long that it is able to reflect off of the wall behind you and cancel itself.

No matter what anyone says this cannot be cured by room treatment using bass traps or room correction using Digital Signal Processing (DSP). In the case of the latter, the cancellation means that you can put nearly infinite power with the same result until the wall behind you collapses. With the former, to be effective the bass traps need to dynamically move about the room as the bass notes change.


The more elegant solution is to use multiple subwoofers arranged asymmetrically in the room. This breaks up the standing waves and you get evenly distributed bass. The trick is to make sure the subs don't go above about 80Hz so as to not attract attention to themselves. Then the main speakers will convince you that the bass notes are coming from them. Audiokinesis.com makes a sub called the Swarm that is meant specifically for this use; its one of the very few I've seen that is designed to be as close to the wall as you can get it and still be flat (to 20Hz), and they are not crazy money.