Main memory is responsible for a large and increasing fraction of the energy consumed by servers. Prior work has focused on exploiting DRAM low-power states to conserve energy. However, these states require entire DRAM ranks to be idled, which is difficult to achieve even in lightly loaded servers. In this paper, we propose to conserve memory energy while improving its energy-proportionality by creating active low-power modes for it. Specifically, we propose MemScale, a scheme wherein we apply dynamic voltage and frequency scaling (DVFS) to the memory controller and dynamic frequency scaling (DFS) to the memory channels and DRAM devices. MemScale is guided by an operating system policy that determines the DVFS/DFS mode of the memory subsystem based on the current need for memory bandwidth, the potential energy savings, and the performance degradation that applications are willing to withstand. Our results demonstrate that MemScale reduces energy consumption significantly compared to modern memory energy management approaches. We conclude that the potential benefits of the MemScale mechanisms and policy more than compensate for their small hardware cost.