I have used passive preamps before and I think you are mistaken if you think it doesn't colour the sound.
When it comes to passive attenuation, the only coloration comes from inferior switching or potentiometers. If resistor ladder is used, the quality of the resistors is very important too. But that is the only coloration you get. :-)
You have to read up a bit more on the Ayre volume control topology.
Oh, I have read about it. Charles has done a good job on it, and the idea is great. However, I don't really like mechanical switches, simply because I could hear them.
That from what I have read and what I have read appears to be the best to do volume control.
Okay, everyone is entitled to their own opinion. As I've mentioned, I do hear mechanical switches/potentiometers, so I use other type of devices and non-magnetic resistors for my attenuators. :-)
Best wishes,
Alex Peychev