Do let us know if you find the problem!

I suppose you know, that even if the wires are all hooked + to +, and - to -, there still might be one out of phase. You might try just taking one speaker and swapping the wires to verify that.

One more possible item - check how the speaker is mounted. It should be on a very firm piece of metal. Also, make sure that it is sealed to some extent, so that the wavw out the back doesn't leak out and cancel the wave out of the front. As an example, you know how bad a raw driver sounds sitting in your hand? There may be a little of that at play too.

Let us know what you find out.