i need to pick a box

Systems

i need help picking a new box. right now i have 2 polk ex series II 12's. i have them in a ported box. i have a kenwood 728s 600 watt amp pushing them. i drive a geo tracker which i have no backseat but my friend wants to sell me his bandpass box. right now, i'm hitting a little around 154.4 db. i got it tested by whitlee when they held a competition down here. would the bandpass box help or hurt me? i'm not worried about having a back seat. also, i was thinking about getting a new amp, what would you reccomend, my speakers hold 400 watts a piece. thank you for any help