A 68-year-old man with long-standing hypertension and type 2 diabetes presents with progressive dyspnoea and ankle swelling. Echocardiography shows severe aortic stenosis with a peak velocity of 4.2 m/s, mean gradient 45 mmHg and valve area 0.7 cm², with preserved LV function. He reports exertional presyncope. What is the most appropriate next step in management?