  The stiff-glass approximation of the mode-coupling theory of the
glass-transition--as introduced in a recent paper by G\"otze and Mayr for a
discussion of phenomena resembling the Boson Peak and High-Frequency Sound
observations made in real glass-formers--is examined in detail. It amounts to a
neglection of two-phonon processes and thus provides a clear physical picture
for the MCT solutions deep in the glass. In addition, the effect of additional
symplifying approximations and the combination of these approximations is
studied.
