In a room 4.8 metres long, 36 Hz is a good frequency to put on your test list. At room temperature, sound at that frequency has a wavelength of roughly 9.5 metres. Half of that fits between the front and back walls. A chair near the middle of the room can sit in a weaker part of that particular standing wave, even if the same note sounds strong nearer a wall.
That is one way to account for a difference between seats. Genelec's monitor placement guide explains how rooms can develop pressure maxima and minima, and how the positions of both the speaker and listener affect the result. It takes a comparison in your room to find out whether that explanation helps with the bass you hear.
Use the other dimensions too
You can get a rough answer with a tape measure. Take a rectangular room that's 4.8 metres long. At the first resonance along that dimension, the distance between the walls is half a wavelength. That makes the full wavelength 9.6 metres. Sound travels about 343 metres in a second at room temperature, so divide 343 by 9.6. The result is just under 36 cycles a second, or 36 Hz to the nearest whole number.
Do the same for the other two dimensions. A width of 3.6 metres gives a wavelength of 7.2 metres and a frequency near 48 Hz. A 2.5-metre ceiling gives 5 metres and roughly 69 Hz. Write the three values beside the corresponding walls and ceiling on a sketch. If your room has a large opening into the hall, mark that too. These are rough predictions for three modes in a rectangle, and a useful frequency might sit a few hertz away from the number you wrote down.
Check from the chair, then from the next seat
Put a piece of tape on the floor for the current seat and another 25 to 40 centimetres forward. Put a third mark behind the chair if there is space. Keep each mark on the front-to-back centreline between the speakers, and keep your ears at the same height. Repeat the same passage at all three positions. Keep the speakers, volume and EQ fixed.
If one low note comes back at the forward mark, check a second recording before leaving the chair there. It is quite possible to gain the lowest note and find the next one booms. You can also use a measurement microphone. Label the traces by position, take them at ear height, and leave the sweep settings unchanged. If a suspected problem near the height mode persists when you move forwards or back, try a separate comparison at a slightly different ear height. Keep the rest of the setup fixed for that comparison.
Adjust one thing at a time
When a dip fills in after a seat change, try living with the new position first. Extra gain at the old position can cost headroom without filling much of the dip, as our power and decibels guide helps explain. A broad bass excess at several seats is a better reason to consider a modest cut. If you can't move the chair, try a different position for one speaker or the subwoofer. Put it back if you lose more than you gain. See Genelec's room acoustics guide for placement and treatment options.