i owned a pair of 2.7s for several years. the best sub for it is no sub. once you mix subs and panels, you lose coherency. the resulting configuration sounds like two speakers. if you must do it, run the nagnepans at full range and hopefully find a sub that rolls of at around 50 or 60 hz, steeply, to minimize the overlap.
try to avoid active crossovers. keep it simple.
try to avoid active crossovers. keep it simple.