It's audible on Horowitz recordings at 30th St. from 1962 to 1973 - not all (eg Liszt - Consolation in E major, recorded in May 1962) but on his 30th St. recordings in the colder months spanning the whole decade you can hear the characteristic rumble... Here's a January 1973 example: Schubert - Impromptu in A-flat major, Op. 142, No. 2 November 1962 example: Schumann - Kinderszenen, Op. 15: No. 10, Fast zu ernst May 1962 - no characteristic rumble: Liszt - Consolation in E Major, S. 172 (R. 12) , No. 2 It has not been removed in mastering either originally or in the modern era.