For what you want to do, with a $300 budget, my suggestion would be to grab that Hifonics BX1000D in the FS forum and then pick up a 10" AA Arsenal. You'd be right there at your budget and if you're wanting good sound with the ability to get loud when need be, I simply don't think you will find a better sub/amp combo in your budget.
*Oops, I just saw that he's looking for a 12 or 15. I don't know why I was thinking a 10. Well, he could go with a 12" Arsenal, but he'd have to bump his budget up by about $50. If he opts to go with an Assassin, the biggest they make is a 12". He's not open to a pair of 10s?