From Prompting to Preference Optimization: A Comparative Study of LLM-based Automated Essay Scoring. - Minh Hoang Nguyen; Vu Hoang Pham; Xuan Thanh Huynh; Phuc Hong Mai; Vinh The Nguyen; Quang Nhut Huynh; Huy Tien Nguyen; Tung Le 004 (CoRR, 2026)