volume displacement determines how loud a sub is. sd x xmax = vd
08s12l74 - 99.98in2 x .56in = 56in3
08s15l54 - 163.68in2 x .67in = 109.7in3
thats almost a 6db difference, every 3 db is equivalent to doubling power. so the numbers say the 08s15l54 will be EXTREMELY louder