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

I own some Soliloquy 6.3's. The speakers are approx 7 ft apart and sitting area approx 8-9 ft. back. When I stand the bass is perfect. When I sit (same distance)the bass seems to be reduced approx 50-60%. Can someone tell me what is going on? I can do no better than getting 18 inches from back wall, large room 20x22 with semi vaulted ceiling. I have not installed the spikes. Using AR Vs110 (100 watt tube) amp. HELP
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.
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!
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).

You are in a bass node,a place where direct & reflected bass energy are "piling on".You can adjust speaker distance from wall behind them,your listening chair foward or back,bits of both and or add bass traps starting with corners behind speakers.I would start out with adjusting placements,which will take patience and time but cost nothing.Bass traps and room treatments should help in all areas of sound such as image focus and harsh or bright reflective energy.There are links to diy basstraps that are very effective and attractive along with other room treatments.You have just discovered that the "room" is a big factor in sound and of major importance to end result.
I must tell you all that I am unable to move the speakers no more than 18 inches out from the rear wall. Married here and well... you know?
As for seating position and speaker distances from each other I can/will...
Any other suggestions?
I love my Soliloquy 6.3's when I am standing but seated, different story, and I mean a BIG differnet story.
Thanks guys,
At least move them out to see how well you can make them sound. Then you'll be in a better position to negotiate.

...but be careful that one of YOU aren't moving out next!
i run sol 6.3s from an ARC VT130 in a large room w/ vaulted ceiling also. i've never been able to get clean base until @ least out from front wall 39 inches. my side walls are each more than 15 ft away, speakers 116 in apart & 9 ft from chair back. the sols have been very sensitive to front & sidewalls for me.
I have my 6.5s 72" out from the wall behind them, measured from the center of the front face of the cabinet. This corresponds to 1/3 of the front to back room dimension and gives the smoothest bass response as well as the best stereo imaging and depth. Spacing is 9' apart, measured from centers. Yeah, they're a little in the way, but they sound gooood.
The speaker might be too big for your room
Too big or too small?
I agree it is a problem, but I believe you can get good bass from those speakers when only 18 inches from the back wall. The measurement to the back wall is just one measurement. There are several others to be played with. You ought to also think about the other measurements to closest boundaries - floor, side wall, ceiling, and the opposite wall. Take those measurements accurately and look for mathematical relationships, such as is the 18 inches to the back wall the same as or twice or half the measurement from the bass driver to the floor, or to the side wall. Using these measurements to shift the bass nodes around the room can even be calculated by available computer programs if your room is cuboid (is that the right word?). If all else fails, get radical about the seating position.

What I am saying is that I have had to cope with this 18 inches to the back wall issue with several speakers before, and been able to find something that works in the bass. The issue that is often much harder to deal with is that 18 inches is barely enough to avoid the soundstage collapsing due to reflections of higher frequencies than the bass. This requires damping of reflections between and behind the speaker, and when the speakers are this close to the wall, making sure there is nothing between those speakers.
Nodes and anti-nodes are a function of spacing between you, the speakers, and the boundaries.

You need to move yourself, your speakers, or the room.

The best locations for bass and midrange/imaging are probably different.