Hi,
Have you run your AVR's room calibration appropriately (from the listening positions)? What did it set for your center?
Your room is on the large side, since it shares space with the kitchen. Sitting 15 feet away from your center channel is going to be demanding of the system if it's not sensitive enough. Your LC2 is 92db/2.83v/1meter and should be able to hit 50hz on the low end, but that depends on room response, placement and crossover. Your receiver is probably doing 60~80 watts per channel when all driven (not the 140 watts which is when only 2 channels are driven). I don't see a white paper on it, so that's just my assumption. With your LC2, at 92db sensitivity, at 15 feet, 1 watt of power is already going to put you at 79db roughly which is above reference. So you should have enough power since your speaker is sensitive with headroom to spare.
If you're having trouble hearing dialog, it's likely calibration and whatever settings you're using. Not the speaker. Especially if you say you switch to stereo to hear dialog. That implies a problem with settings.
Your speakers are fine. You just need to get more into the setup, how the AVR works, how the speakers work, what all the stuff means, how to set it up, how to measure things, room response effects, acoustic treatment, what level of volume you listen at and how that is effected and performed, etc. Just deep dive into the hobby a bit so you can make better purchase decisions, or better yet, the realization that you don't need to purchase anything and it's all setup based for this particular issue you're having most likely.
Very best,
