Im building a new isobaric box for my 4 15s next week! im firing them up and down front to front with 2 ports firing back. not sure on the tuning yet because i have to run all the numbers into the computer. dont be jealous guys when my 4 15s take up just as much space than your 2 15s...haha.
It was the best solution to saving space and not loosing too much money on buying new subs. Plus, ill hopefully get much more SPL than i would with 2 15s. Box design will be a little tricky but hopefully it will be worth it in the long run.