Cheap amps produce dirty power where the output of the amp is clipped or distorted. The clipped wave causes the sub to not be in motion for a very small frame of time. The coil on the sub will heat up faster because the sub isn't moving and cooling itself off, and after enough time the coil will separate and you have a blown sub.
You can get Level 3's built up with level 4 soft parts that will make them take 1k without issues