The level of detail of a work statement is understood in the context of the difference between the customary statement of work (SOW) and performance work statement (PWS). The PWS is based on the concept of performance-based acquisition (PBA), which aligns with the part of your question "...outline the range of requirements without the need for a SOW." FAR subpart 37.6 covers PBA and what it entails. However, FAR 37.102(a)(1) states: "Use performance-based acquisition methods to the maximum extent practicable, except for [emphasis added]- (i) Architect-engineer services..." In other words, FAR policy is for A-E services to be acquired using a detailed SOW rather than a PWS. The Army Corps of Engineers awards many A-E contracts. For additional guidance I recommend you perform a search on "statement of work" in its publication ARCHITECT-ENGINEER CONTRACTING IN USACE.