are you using win isd? basically the graph you view is in theorey correct but it's not so. There is a peak "loudness" to every box. If all we had to do to make a sub loud was increase boc size then we should have records way higher than what they are now. Try to stay within the manufactuers recommended box sizes unless you are going for straight spl. If those 15's are designed for large boxes like that then yes you will gain some output. or at least you should.