If you're set on a separate Hi/Lo pass filter and amp, the Dahlquist DQ-LP1 is an excellent unit. The Hi-Pass circuit consists of only one capacitor(as high a quality as you choose to employ), chosen based on the input impedance of the power amp. Be certain to find one with the manual
included, as it contains the instructions/chart for figuring out the value of the cap. The RCA ins/outs were pretty cheesy, but- can be easily replaced with WBT connectors. Dahlquist is no longer in business, but- the guys that worked there have another company(Ragnar) and still support
their designs. (http://www.regnar.com/dahlquist-dq-lp1-parts---upgrades.html) They can be hard to find, in great shape, but- worth the effort. ie: (http://www.ebay.com/itm/Dahlquist-DQ-LP1-Continuously-Variable-Low-Pass-Filter-Excellent-/231688602431?hash=item35f1b7833f) or (http://www.ebay.com/itm/DALQUIST-DQ-LP1-Stereo-Crossover-with-all-the-Manuals-/221779357315?hash=item33a3146e83)
included, as it contains the instructions/chart for figuring out the value of the cap. The RCA ins/outs were pretty cheesy, but- can be easily replaced with WBT connectors. Dahlquist is no longer in business, but- the guys that worked there have another company(Ragnar) and still support
their designs. (http://www.regnar.com/dahlquist-dq-lp1-parts---upgrades.html) They can be hard to find, in great shape, but- worth the effort. ie: (http://www.ebay.com/itm/Dahlquist-DQ-LP1-Continuously-Variable-Low-Pass-Filter-Excellent-/231688602431?hash=item35f1b7833f) or (http://www.ebay.com/itm/DALQUIST-DQ-LP1-Stereo-Crossover-with-all-the-Manuals-/221779357315?hash=item33a3146e83)