15" xxx and a 12" kappa perfect? well i guess if you gave the the xxx a incredebally low tuned port then made a TL for the kappaI know for a fact I could make a Perfect almost as loud as a XXX both ported. But I'll stick with my XXX.
now if you were fair and designed the loudest possible box for both of them (assuming TL not that its practical or anything) the xxx would come out on top with no second guesses
and just so you know, i personally dont think you could get that single 12 anywhere the near the loudness of the xxx
