As Ralph Karsten (atmasphere) has numerous times said here, a preamp-less system can work well IF the source component is up to the task of driving the power amp directly. If the source is NOT up to that task, the system may actually sound better with a pre-amp, as imperfect as it may be, inserted between source and power amp.
What are the requirements for a source component to be up to the task of driving a given power amp? Ralph has more than once listed them; hopefully a search of the Forum will bring up the related threads.