Have to agree with Herman, the topic seems to come up once a month.
When discussing passives you need to break them out into a few different camps. There are those that use resistors to attenuate the volume, those that use transformers, and those that use autoformers (a transformer with a single winding). Within the resistor camp there is a sub group that used opto-couplers in the design, thus eliminating any effect the switch may have on the sound.
While the benefit of a pure signal at a fraction of the cost of an active preamp is an ideal goal of a passive preamp. The reality is some cost more, much more. Audio Consulting is a good example. Not only that, but passives, while providing a high level of transparency, can induce their own coloration and sonic signature. One example would be Audio Consulting's copper and Silver Rock transformers. The silver wound sounding more detailed and transparent IMO. Another example were S&B transformers of which there were the MkI - MkIII series. I preferred the copper MkI versions of these transformers as there was just a touch of warmth added to the sound. Resistor colorations are also possible - Vishay, Mills, Caddock, Tantalum, etc. Then there are the switches.
So all in all, while we hear how transparent and pure the signal can be with passives, there are some caveats to that. For me, the big issue is reduced noise. I also feel the level of transparency cannot be beat versus actives. If you have enough gain from your source and the rest of your system is passive friendly then a passive preamp may make sense. However, if you're someone who prefers coloration and what some will say is more "weight" to the sound (maybe its just another form of pleasing distortion), then I doubt a passive will be your cup of tea. Better to "tune" your system to you liking in other ways.
In my system the passives I have/had are:
S&B MkI TVC (custom built by K&K Audio)
Lightspeed Attenuator (boutique unit built by George Stantschleff)
Slagleman Autoformer Volume Control (prototype built by John Chapman)
The S&B was used with the solid state TRL D-225. The other two have been used with VAC Auricle Musicblocs, VAC Vintage Williamsom 35/35, and now a Music Reference RM-10 MkII. All of these passives worked fine with the respective amps.
