A 62-year-old man presents with progressive exertional dyspnoea and orthopnoea. He has a history of long-standing hypertension and past myocardial infarction. Echocardiogram shows an ejection fraction of 25% and moderate mitral regurgitation. Despite ACE inhibitor, beta blocker and spironolactone, he remains NYHA class III. What is the next evidence-based pharmacological addition?