A 68-year old man who has had a recent syncopal episode is hospitalized with congestive hea failure. His blood pressure is 160/80 mmHg. His pulse rate is 80 beats per minute, and there is a grade III/IV harsh systolic murmur. An echocardiogram shows a dispropoionately thickened ventricular septum and systolic anterior motion of the mitral valve. Which of the following findings would most likely be present in this man :