A good deal of LP rumble is not "subsonic". Although a subsonic filter will prevent your amplifier power being wasted, it may not solve the audible problem you are having.
You might consider the Audio Control Richter scale (several hundred dollars) which has a good subsonic filter, as well as a 1/3 octive equalizer covering the range from about 150 Hz down. It also has an electronic crossover section, in case you ever want to biamp, or subwoof.
