What is a preamp? The basic description is "a device that serves as an input selector and router between the source and a power amplifier, that can attenuate or amplify the overall volume level."
A Preamp can be a simple passive signal router with no power supply or active circuitry or it can be a complex multi-box multi-voltage device with highly sophisticated active circuitry. In either case the preamp does the same thing - get the signal from point A to point B with as little sonic deterioration as possible. The perfect preamp is "a straight wire with (or without) gain." That's a phrase I lifted from Stewart Hegeman, who was the guy who designed the legendary Citation line of tube electronics for Harman Kardon in the early 60's.
Some audiophiles, especially those who have a room-based rather than desktop systems, sometimes notice that passive preamps don't deliver adequate dynamic contrast because they don't have adequate buffering between source and power amplifier which causes impedance mismatches that can suck the dynamic drive out of the music. My own experiences with passive preamps with room-based systems is mixed. Well-designed passive amplifiers, such as the Reference Line or Placette controllers were made to supply buffering. I've used the Reference Line in countless systems over the years with no negative effects. But if the cable length is long enough, say over 30 feet, even a well-designed buffered passive preamp might run into trouble. That's where active preamps are the way to go.
If one of your music systems is a desktop computer-based system with short cable distances of one meter or less between components a passive preamp is an ideal choice. I've never experienced any dynamics or gain issues using a passive in my high-end desktop systems, and I've tried many combinations of DACs and power amplifiers over the years....
The only thing better than a passive preamp in a desktop system is no preamp at all. Many USB and Firewire DACs have built-in pre-amp functions. The Wyred4Sound DAC2 , Bel Canto DAC 3.5 VB, Benchmark DAC HDR, and the Weiss DAC 202 all have excellent attenuation systems that make an external preamp unnecessary.
Merely getting rid of the extra cabling and connectors (it takes an additional set of interconnects to put a preamp between a DAC and an amplifier) in a desktop system results in better overall sound regardless of what preamp you remove from between a DAC and power amplifier. That's why I strongly advise anyone considering a USB or Firewire enabled DAC to look at units which include a preamp function. Less can be more, especially when it comes to pre-amps.