If you want a bass cab with two 15" speakers start out with your amp. Can you put out 400w into 4 ohms? Then use two speakers in parallel if thay are 8 ohm speakers. If not other pathes of lojic will persist. Close back is a good idea but one or two 4" ports arnt a bad idea eaither. Ported frount is not a bad idea but if the back is allready ported than porting the frount is a waist of time.