I installed a Marchand fully-balanced passive 80 Hz 24 dB/octave high-pass filter between my KX-5/20 and VX-5/20, adjusted low-pass of a pair of Velodyne HGS-10s from 40 Hz to 80 Hz 24 dB, and ran the SMS-1 acoustic room correction. The change in sound is subtle, but at least the quality is not degraded by insertion of the passive filter. I'll do more listening, but I suspect just supplementing the KEF Ref 1s with 40-45 Hz low-pass 24 dB is sufficient.