Systems that provide QoS-enabled services such as multimedia are subject to constant evolution - customizable middleware is required to effectively manage this change. Middleware services for resource management such as scheduling, protocols providing security and reliability, load balancing and stream synchronization, execute concurrently with each other and with application activities and can therefore potentially interfere with each other. To ensure cost-effective QoS in distributed systems, safe composability of resource management services is essential. In this paper we present a meta-architectural framework for customizable QoS-based middleware based on the actor model of concurrent active objects. Using TLAM, a semantic model for specifying and reasoning about components of open distributed systems, we show how a QoS brokerage service can be used to coordinate multimedia resource management services in a safe, flexible and effcient manner. In particular, we show that a system in which the multimedia actor behaviors satisfy the speciffed requirements, provides the required multimedia service. The behavior speciffcation leaves open the possibility of a variety of algorithms for resource management as well as adding additional resource management activities by providing constraints to ensure their non-interference.