No, your speakers are most likely not defective. A speaker's placement in the room has a huge effect on what it will sound like from any given listening position. The Audyssey MutiEQ process does take audio samples from multiple positions in the listening area, so it should get a more accurate picture of a speakers performance, but the effects of the room cannot be downplayed.
That being said, I'd experiment with manually overriding the cutoff frequency on your sides are rears by setting them to 100Hz. If you think they sound tinny, this probably won't help because it will simply move some of the bass from the sub channel back to the side/rear surrounds.
