A year or so ago, in my review of the Pass Labs XP-30 preamplifier, I wrote that the heart of an audio system is the preamplifier, in that it sets the overall quality of the system's sound. But it is the power amplifier that is responsible for determining the character of the system's sound, because it is the amplifier that must directly interface with the loudspeakers. The relationship between amplifier and loudspeaker is complex, and the nature of that relationship literally sets the tone of the sound quality.