Home stereo system?????

i was wondering wat would be a nice and loud stereo setup i allread have a onkyo home theater system at 330 wats hooked to a 6-8inch woofer and 4 smaller speakers im looking to get 2 12's or 2 15's and i just wanaa make the windows rattle wat do u think would give me the most power for the least cost......and i seen a jbl 12 that says built in 250wats does that mean its allready got 250 +wat ever u hook up to it????????????????