resonance problem

I was wondering what are some common causes of resonance. I have a set of Paradigm Monitor 70p's and one of the speakers resonantes whilst playing some of my favorite piano tunes. I took my speaker back and the tweeter diaphram was replaced. Dispite this repair the speaker still resonated, but to a lesser extent. I took the speaker back in for repairs after I showed the tech what the problem was and let him listen to the speaker. It was suggested that the amp be replaced, as they are powered towers.I was told that the speaker was sent back to the factory for repairs. Could it be that this one speaker was a bad one that got out in the world with a defective cabinet and/or drivers?

Follow Ups: