An exemplary two-stage prosthetic heart valve system has a radially expandable base stent implanted within a native valve annulus. The two-stage heart valve system also has a valve component that is delivered to and mounted within the base stent in a separate or sequential operation after the base stent has been anchored within the annulus. The valve component in certain embodiments comprises a hybrid valve component that includes a conventional, non-expandable surgical valve that is modified to include an expandable coupling stent that can be expanded to engage the inner surface of the base stent, thereby anchoring the valve component to the base stent. In its expanded configuration, the outflow end portion base stent has tri-lobular shape that closely conforms to the shape of the aortic root and a support ring of the prosthetic valve that is mounted within the base stent.