HUMMEL: SONATAS NOS. 1-2, 5-6. Ian Hobson, piano. Arabesque Z6564/Z6565 (two compact discs). As teacher and performer, Johann Nepomuk Hummel's reputation far exceeded his life span (1778-1837). Hobson's sterling performances of these four piano sonatas (Arabesque promises the entire cycle) may boost his stock as composer, too. You can always play a game of spot-the-influences: the second sonata wavers shamelessly between the rococo brilliance of Haydn and an uncanny anticipation of Chopin's gloomy grandeur, while the meter shifts and heavy chords of the sixth sonata bring Hummel to Robert Schumann's door. But on his own, the composer often abounds in crafty pianistic invention, even if he adheres to less than a towering sense of architecture. Hobson's decision to perform on a Hamburg Steinway firmly anchors the subject of his labors in the 19th Century, but, on its massively scaled (and fastidiously recorded) terms, Hobson's playing has much to commend it.