In his Aug 7 blog Paul McGowan of PS Audio asks the question, "Is There a Perfect Preamp?" which he never gets around to answering completely. I'll answer for him. No, there are no "perfect" preamps...why is that?
The problem with analog preamps is that to make one that does the least damage to the signal requires a lot of high-quality parts arranged very carefully to minimize hum and noise. All these parts and a designer's time to carefully arrange them costs money. And manufacturers can't resist putting all these expensive parts into a fancy case, which further increases the final pricetag. In short, there's currently no way to make a high-quality analog preamp inexpensively.
To further increase the degree of difficulty in achieving complete preamp transparency, even the connectors and wire used both internally and externally to attach your sources have a negative effect on transparency. During my time as an audiophile I have yet to hear an analog preamp that passed a bypass test - that involves merely placing it into the circuit path at unity gain. Every time I've done this test I've detected some amount of reduction in transparency. No, analog preamps are NOT transparent. So, what about an all-digital preamp?
There's hope that an inexpensive all-digital and completely transparent volume attenuation and augmentation is possible. Some manufacturers are claiming to already have such circuits in their DACs, but there's still the issue of conversion of analog sources to the digital domain - Analog to digital convertors still have negative affects on transparency - and even the newest wave of PWM digital amplifiers need an analog to digital convertor for analog sources.
Circling back to the original question - the reason there are no perfect preamps is that analog preamps are incapable of complete transparency and the "perfect" digital preamp (at all volume levels) has yet to be implemented. But there's always tomorrow...