What amp you running at the moment on that one Memphis 12?At a $400 dollar budget, it can be done, but you may have to consider a used amp,new sub/subs, and build your own box,with a cut out sheet possibly from a box builder on here.It can be done though..$400 bucks..wont be top notch top grade stuff, but will get your moneys worth for the budget,that i will try to recommend you