User blog comment:Uraharattack/Starrk's classification as a hollow?/@comment-1387259-20130820163125/@comment-8295743-20130821075530

@Sternritter M, no. Most of the Espada before joining Aizen were still Adjucha-class. The only exceptions were Harribel, who was stated as being a Vasto Lorde, and Aaroniero, who was a Gillian-class. Staark may have removed his mask of his own power, it's not really stated irrc. Isshin stated that the Shinigami had encountered Arrancar many times in their history, but they were rarely an actual threat, as most were still basic Hollows. Even a low-ranking Arrancar created by Aizen was shown as being stronger than Harribel as a Vasto Lorde, and Hitsugaya believed the VL-class were stronger than the average Captain.

Arrancar were certainly around before Aizen was, but most of the Espada were shown as being Adjucha class Hollows, who still had their masks. The power difference through Aizen, however, was much greater than anything the Shinigami had faced before, from a Hollow-based enemy.