What should i tune my box to for 3 JL 12w6s..dual 6's old schools...my box will be about 7 cubic ft...6.76 something...ported box with port on the back facing rear of car.. with subs facing up..i have an SUV...thanks..have not built box yet just sketches...JL says i need to tune each to 27hz...i think that is wrong..seems unlikey but its what they say...but for 3 i have no clue..never doen 3 subs