for 15's, depending on power, i use 3-5 cubes per 15. i had my old 15's in about 4 cubes per sub with ~12 sq inches of port per foot. and i personally wouldnt tune to 29 hz, i find it pointless. i stay in the mid 30's. you're only going about .5 cubes bigger per sub after displacement (give or take .5 for displacement), so i think you'll be fine