This article investigates aspects in the context of the incremental software development, i.e. software product lines. Specifically, we propose the integration of aspects into AHEAD, an architectural model for feature-based product line development. We introduce the notion of aspect refinement based on Aspectual Mixin Layers, a novel technique for implementing features. Aspect refinement enables a programmer to evolve aspects over several product line development stages. This is novel since common AOP approaches do not have such an architectural model. We realize the idea of aspect refinement by introducing mixinbased inheritance to aspects. Furthermore, we propose bounding quantification that reduces the complexity and unpredictability of aspects in incremental software development. Our novel bounding mechanism exploits the natural order of the layered architecture introduced by the concept of aspect refinement. Aspect refinement and bounding quantification improve the incremental development of product lines using AOP techniques.