Lv = Av*1.84*10^8/[Vb*1728*(Fb/0.159)^2] - 0.823*sqrt(Av)
Lv is port length
Av is your port area = 93 (15.5 x 6)
Vb is the internal volume of your box = 9.01476
Fb is the tuning frequency you want = 28 Hz
I get a port length of 35.0725, is that correct? or did I fu(k this up too?