They have a big section dedicated to the differences in sound between the various classes of amplifiers:
Class A Best attainable sound for a component of its kind, almost without practical considerations; "the least musical compromise." A Class A system is one for which you don't have to make a leap of faith to believe that you're hearing the real thing. When SACD and hi-rez files became available, we introduced a new Class, A+, for the best performance in those digital categories. Class A now represents the best that can be obtained from the conventional 16/44.1 CD medium. We also created Class A+ categories for turntables and phono preamps, to recognize the achievements of the Continuum Caliburn and Boulder 2008, respectively.
Class B The next best thing to the very best sound reproduction; Class B components generally cost less than those in Class A, but most Class B components are still quite expensive.
Class C Somewhat lower-fi sound, but far more musically natural than average home-component high fidelity; products in this class are of high quality but still affordable.
Class D Satisfying musical sound, but these components are either of significantly lower fidelity than the best available, or exhibit major compromises in performance—limited dynamic range, for example. Bear in mind that appearance in Class D still means that we recommend this product—it's possible to put together a musically satisfying system exclusively from Class D components.