i got 2 mtx 5500 15's ina down fire box below window line in my truck, i think its 4 cubes and it slams hard, ppl dont kno wtf is in it, i like downfire more cause it reminds me of how a car would act if the subs was faceing back, just make sure they are around 3'' off the ground if u build ur own box or the sub will hit the floor, my box has 4 chambers in it, 2 lil ones on the ends so i can run my wires threw and use my 6&1/2's as rear speakers, and 2 in the middle sealed devided wit the 15's in it, just make sure the deeper the box the deeper the sound, i found that out the hard way, but to big of a box and they sound like shit, and my buddy kevin put 2 of the pioneer shallow 12's ina lincon truck powered wit a mtx1501d and that shit was loud and hit mad lows, ied do the down fire it saves space and would be a bitch for someone to steal my subs, its gunna take me bout 6hrs just to change mine out to my mtx 8000's, and it gives a more cleaner look to, but if i was u ied make a trim piece wit a grill over were the sub sets off the ground so nothin goes back there and gets beat all to hell by the speakers, hope this has helped u in ur jorney on down fire sealed boxs, i was wantin to do a ported down fire but doesnt make much sence, mtx has some prebuilt for suv's to go behind the 3rd seat, but anywho thats my $.02