Hi Adrian,This looks like a great cabinet to me. The only thing I would mention is you say 70 liter for the lf driver. You have the whole cabinet volume, less the horn displacement available for internal volume. If your cabinet are 14" deep I calculate about 160 liters internal. Less the two horns should still net you 125+ ?
With horns it is not necessary to close off the lf driver in a chamber. JBL and all others I have seen utilize all the cabinet volume surrounding the horns for the bass. You need to build a closed chamber around your mid horn driver is all.
Bill