How it is possible to make bad audio amp?
The use of opamps in amplifiers can drastically simplify their design, but opamps aren't perfect. If they would have infinite amplification over their full bandwidth they would tend to oscillate, so they are internally compensated, which limits their bandwidth. A limited bandwidth makes the amplifier prone to Transient Intermodulation Distortion (TIM), a type of distortion much more annoying than harmonic distortion (HD).  


  The reason only HD is published, and TIM never is, is that it's much easier to get good-looking HD figures. Who wouldn't be impressed by a figure like 0.01% harmonic distortion? Most customers don't realize that this figure is totally irrelevant because the total system's distortion is for the most part determined by the speakers, which easily add a few percent distortion.  


The power stage isn't without its problems either. Class A amplifiers are hardly used because of their low efficiency. Class B or AB amplifiers have a crossover distortion where one transistor takes over from the other. This is a non-linear distortion which can't be compensated by feedback. May not be true. If someone can enlighten here I'd love to hear it..  

A final quote on opamps:


  "There is no such thing as an unconditionally stable op amp unless it lies on the table with power disconnected" [1]  




Further reading
[1] Intersil appnote AN9415: Feedback, Op Amps and Compensation