The aim of the present article is to render
the spectral theory of mean values of automorphic $L$-functions --
in a unified fashion.
This is an outcome of the investigation commenced with the
parts XII and XIV, where a framework was laid on the
basis of the theory of automorphic representations and a general
approach to the mean values was envisaged. We restrict ourselves to
the situation offered by the full modular group, solely for the sake
of simplicity. Details and
extensions are to be published elsewhere.