No one type of preamp is going to be ideal for every system. There are many variables to consider such as the type of source being used, gain of the amplification and output voltage or gain of the preamplifier.
We all have our views on passive vs active and this subject will be debated till the end of time but one thing we will never get away from is system synergy and choosing components that work together properly for "our" personal system requirements.
I, for one, believe providing such specs is important for the consumer to make better buying decisions.
We always show all the specs on our Musica Bella Audio preamplifiers and have always offered to customize to the personal requirements of any given customer's system.