Some older Japanese integrated amplifiers used to have rumble filter switch. I had a Kenwood amplifier from 1978 that had a switch labelled "Subsonic" and the manual specified -10dB/octave starting at 20Hz. The problem with all analog component filters is that they introduce frequency dependent phase shift and the sharper the cutoff the greater the phase shift. Rumble used to be a problem with old 78 rpm hard shellac records played on older turntables with relatively imprecise bearings. Modern turntables playing 33 rpm LPs made of PVC have rumble specifications that are trivial.
Why don't all phono preamps include a rumble filter?
In my system, the use of a rumble filter is mandatory (I notice no degradation of sonics when I engage the rumble filter). I'm thinking of upgrading my phono preamp and would like to choose one that has this as an optional feature. However, it appears that most manufacturers of phono preamps do not include this. Why not? Does the use of a rumble filter degrade the sound of a phono preamp?