Right now I am shopping for another sub for my car. Currently I have one 15" 400w Rockford sub and I'm looking at a 12" rockford 1000w sub. Does anybody know if having a 12" and 15" together would sound ok?
It won't have the best sound if you use it on the same aplifier. It will however be louder so if you just going for that you will be ok.
You could always run them on separate amps and depending on the sub you could use the 12'' (because its smaller) as a midrange. Turn up the subsonic filter on your amp if you have one and use rear RCA jacks.
But overall I would say use the 15'' and sell the 12'' and buy some components or something.