A 72-year-old man with a 3-year history of BPH-related LUTS has been on tamsulosin 0.4 mg daily for 18 months with partial symptom improvement. Repeat uroflowmetry shows peak flow of 10 mL/s (minimal change). Transrectal ultrasound reveals prostate volume of 52 g. PSA is 2.8 ng/mL. He denies acute urinary retention. What is the most appropriate next step in pharmacological management?