I believe there is just one speaker behind the oval shape classed as a 22w "subwoofer" believe it of not and it is not necessarily easy to tell if it is working or not.
I have noticed previously there does not seem to be a great deal of action there myself. Since you have mentioned it I have been listening and it is evident there is sound from the whole surface in a way. It seems most evident when you get something like Kerang on and wait for a heavy beat you would expect a subwoofer to show up. Listen at the top and then at the front of the set. The front speakers do not replicate the heavier beats.
In summary, I do not think you should hear any more noise from the left or the right of the speaker. I do not think the speaker is great but it would be missed if it did not exist.
Let us know what you find in the light of the above.