You NHT speakers probably have two sets of binding posts on the rear attached with straps. If that is the case removing the straps seperates the HP and LP crossovers inside the speaker. You don't need an external active crossover unless you bypass the internal crossovers inside the speakers. You can send full range signals to each of the binding post sets and the internal crossovers will filter and pass on the appropriate frequencies.
Which model NHT's?