We have developed a 3D sound system for headphones that allows real-time sound source and user displacement in a virtual acoustic environment. Because of a flexible design that uses different sets of pre-selected, physically modeled filters, the complexity level of the simulation can be chosen, making the system adaptable both to available CPU power and to application requirements. No extensive signal processing knowledge is required in order to select the appropriate simulation complexity. A preliminary evaluation involving 4 users showed that the system provides a satisfying localization of sounds and users (even with limited memory and CPU power) while also giving access to low-level control over simulation complexity. Keywords 3D audio, virtual acoustic environment, interactive applications, physical modeling.