Under powering is as bad as over powering

Just because a sub is capable of running 150 watts doesn't mean you should run 150. In fact a lot of that depends on if you are running clean power or dirty power. My guess is since you are going cheap all around is that you have no crossover to clean the signal up so you are most likely cranking up the gains on the amp and running it straight to the sub. Also most likely is that you are running it full range into the sub and causing even more damage.

Do some research into how to run clean power and what RMS ratings actually mean before you do something dumb for a third time