yeah ive 2 10' fosgates, and on the 2 12, and the 1 15 setup, lots of money and lots of un necessary noises caused by the speakers beating eachother...on teh speaker handlebarsfsr mentioned, to run it properly you youll at least 2 amps, and amps are expensive when you get to the 900+ watt ones....and having different sized subs isnt a good idea...just for the reason your friend said....and the whole back of car will be subs and thats no fun...so go out and buy 2 12" HE fosgates and youll have a Fuggin loud system....also if you get all those speakers, then a) youll prolly, actually will hae to upgrade your cars electrical system, youll have to have at least 2 amps(1 for the 12's and one for 15) the amps alone will run around 1200$$, and with that much bass, youll be wasting have of it, becuase you wont be able to stand it, my friedn had 2 12" in his cherokee, and honest to go it hurt every time it hit...i could honestly feel my skin moving...so 2 fosgate HE 12 inch, will run about 400, maybe even with a box, though doubt it...and one 650 watt bridgable amp will be plenty...also what deck you have/plan on getting, becuase that factors into this equation greatly as well