This English pianist begins a new series called "Preludes, Etudes and Variations - Downtown/Uptown," six programs he will present at SubCulture in Greenwich Village and Merkin Hall. The concerts feature works in each genre by 19th and 20th century composers, plus three premieres. On Tuesday, Mr. Hobson will perform Chopin's "Trois Nouvelles �tudes" and �tudes (Op. 25); Rachmaninoff's Preludes (Op. 23) and the premiere of Stephen Taylor's "Variations."