Successful use of a passive preamp is system dependent. In a highly resolving system the passive should be more accurate to the source signal. The ideal "passive preamp" simply attenuates the source signal without capacitors or transistors (which change the sound). Therefore the signal retains more of its original flavor and sounds less processed and more pure. Loss of bass and dynamics is a due to a mismatch between output level and input impedence.
Ideally, the attenuator should be built into the source component circuit for the most accurate sound. This is because source signal does not have to travel through simply another component (less circuitry), and there is one less connection, and one less interconnect. It appears that some manufacturers realize this, like Audio Aero and Resolution Audio.
