I have 3 favorite preamps:
1. Aikido (designed by John Broskie) - I like it for it's being quiet, clean sounding (but not clinical), transparent and dynamic aside from it being a tube based preamp.
2. Lightspeed Passive Attenuator (Using Light Dependent Resistor "LDR") - No sound coloration. No noise/distortion since the signal does not pass thru any mechanical device, unlike potentiometers where you have the wipers in contact with each other every time it is operated. Very clean sounding and transparent - you hear only your source. It's like a "straight wire approach", as they say since the singal only passes thru the wire and not to any mechanical device.
3. Aikido with Lightspeed as the volume controller
In the future, I hope I can buy or DIY a Tortuga passive preamp:
http://6moons.com/audioreviews/tortuga/1.html