Good sound quality but short life

I had this sub for 5 months in a sealed enclosure and powered by an sundown sfb600, at 1ohm. I have the amp set at about 250w(16v) output for the first 2 months to break in, the I turned it up to 400w(20v) output at 50hz. It lasted a few months and now at lower volume you can hear a vibrating/rattling noise coming from the sub. This is my first mtx sub and I figured the quality would be as good as their amps, but it's not