Hi folks need some info on what might be causing one of my Monitor Audio 705 speakers to pop.

Diagnostics to date.
1) Speaker pops when driven fairly hard, but within reasonable limits. Especailly on Bassy songs/movies such as the Who, Baba Oreilly.

2)Switched the speakers and the poping noise stayed with the speaker so this would indicate that it is not a receiver or amp issue.

3) Completely swapped out the speakers with a pair of Polk speakers and no popping noise occurred. Just really dull, flat and un-detailed sound.

Replaced both drivers in the speaker but still have issues. Getting ready to replace tweeter and re-test.

One guy said that the speaker may have an air leak????? any one ever heard of such a thing?

Any input would be helpful.