The excellent First Watt B4 retails for $1500, but Reno Hi-Fi sells it discounted. It provides a lot of flexibility: 1st/2nd/3rd/4th order filters in 25Hz increments from 25Hz to 3200Hz. All discrete components, no Opamps or IC’s.
But if a 6dB/octave filter will suffice, the "cap-on-the-amps-input-jacks" method is the most transparent way to go.