If a 1st order (6dB/octave) high pass crossover is steep enough for you, all that's required is the installation of a capacitor on your power amp's input jacks. The value of the capacitor plus the amp's input impedance determines the "elbow" frequency of the resulting filter. The formula for determining the capacitor value needed to achieve the desired frequency can be found via a Google search.
If you need a steeper slope (2nd, 3rd, or 4th order---12, 18, or 24dB/octave), you'll need to go active. Pass makes a great one, but it's not cheap. Bryston's is somewhat lower priced, but Nelson Pass makes an economical one (retail $1500) in his First Watt B4, a great little unit featuring 1st, 2nd, 3rd, and 4th order filters in 25Hz increments from 25-3200Hz, for both high and low pass, The filters are implemented via discrete components, no IC's or Opamps. RCA jacks only, no XLR/balanced version available.