The only superseding authority? Really? ADs certainly overrule anything else - they are based upon the latest design data available and are not only approved by the regulator, but are also issued directly by the regulator.
I agree that for something concerning engine power runs and other work related to the engine as installed on the aircraft - where the engine QEC build up includes customized aircraft system related equipment and the controls are certified to the particular airframe type - the AMM has priority over the engine manual, but not for questions regarding the build up and internal inspection of the bare engine.
Airworthines Directives and Service Bulletins containing an approval statement (i.e. the majority) are based upon drawings and empirical data produced by the manufacturer and approved by the regulatory authority - approved data. The AMM and other such manuals are not 'approved documents'. The answer remains as given previously - in the event of a conflict between maintenance documents the proper procedure is to refer the matter directly back to the type certificate holder.