The LX has the widest vertical dispersion of any speaker we offer, depending on your floor and ceiling, you could be getting reflections. However, based on your description of what you are hearing, I doubt this because you are describing the sound as being "thin" at your main listening position which better correlates with destructive room modes because they affect bass and midbass frequencies.
The LX has the deepest bass extension and the most bass energy of any speaker we have ever manufactured, they are, by far, the fullest sounding speaker we offer. That additional energy could be exciting those modes differently than Sierra-1 or Sierra-2EX, which have considerably less bass energy.
So -- since you have a receiver with Audyssey, I recommend using Audyssey to take measurements of each speaker with the mic properly positioned at your exact listening position and height.
Let's take a look at what is going on.
A quick fix to get the sound you want might be flipping the speakers 180 degrees vertically so that the tweeter is on the bottom and woofer is on the top. No problem at all doing this, this changes the position of the woofer and rear port relative to the floor and ceiling height, and your ears.
Also, what are the dimensions of your room, including ceiling height?