Standing vs. Seated-Big Difference in Bass???

just joking but turn them upside down.

Bass Note waves are very long and so nodes and anti nodes are created that mean the bass is diminished or absent in certain spots or enhanced or even doubled in others. You seem to be seated in an anti-node point. Standing up fixes it, but other possibilities are moving your seating position forward or back. Other parameters to play with are your speaker positions. The good news is that you get good bass somewhere which means you have to just work with your room, not ditch either it or your speakers. Sometimes a few inches of movement in each speaker plus your seating position is enough to shift nodes and antinodes quite significantly, so you just need to experiment. Don't bother with the speaker spikes till you get the amount of bass right. If nothing quite works, try setting your speakers up against another wall, or setting them up slightly off-set in the room.

When you stand up you listen to a reflected bass.
You'll definitely need to either kill the reflections with room treatments or tweak the speaker placement closer or appart from rear/side walls.

You do not mention how far out from the wall your speaker's woofers are from the wall behind them. If they are not located at least 5.5 ft out from that wall, then I would recommend trying anywhere from 5.5 to even 8 ft out (measured from the woofer's front).

-IMO

Uh...also exacerbated by the big suckout in the upper mids/low treble when you stand up into the "crossover null" cancellation of the mid and tweeter, allowing your ear/brain to PERCEIVE more bass. Almost all the bass you hear is reflected energy, as the wavelengths are long and pile up weirdly, btw. You'll have to play with listening seat position as well as speaker position to arrive at a smooth response that provides reasonable bass linearity and mid/treble response that's not too affected by sidewall reflections, for example. Keep at it...the rewards are abundant!