I've had good results with Auralex SubDudes under the subs for isolation.
My experience with high-pass filters may be Ayre-specific, but I tried a Bryston 10B active filter and a Marchand fixed passive filter (4th order 80 Hz), both claimed to be balanced. The Bryston really changed the sound, the Marchand removed a bit of the sense of air. Both are gone. I let the KEF Ref 1s run full range and supplement them below 40 Hz with pairs of Velodyne HGS-15s and HGS-10s and SMS-1 bass managers that provide acoustic room correction.
My experience with high-pass filters may be Ayre-specific, but I tried a Bryston 10B active filter and a Marchand fixed passive filter (4th order 80 Hz), both claimed to be balanced. The Bryston really changed the sound, the Marchand removed a bit of the sense of air. Both are gone. I let the KEF Ref 1s run full range and supplement them below 40 Hz with pairs of Velodyne HGS-15s and HGS-10s and SMS-1 bass managers that provide acoustic room correction.