your amp might not be able to handle your sub if you got a 12inch or 15inch sub and you only have a 400watt amp it wont be able to handle it cause the sub is too strong and also if your gonna put a 12 or 15inch sub in your car you might wanna get 1000watt that does the job anything under that some might do the job but you'll be spending more money on buying the amp or sub when you could of just got a good amp mono block 1000watt in the first place i did the same thing and i always had to go out and buy amps every fort night cause i cant listen to music with out subs so i pumped it with 500watts and i blew the amp fuse