dude, feel free to disregard my advice, but you really need to listen to these things to know what will work for you. You can get everyone on this board to tell you that lundahls are the most awesome transformers and they are super clean and totally rule, all of which I agree with, and even though you want a "clean" mic pre, they may be completely the wrong transformer for you. It is possible to have a clean sounding pre while still having tons of transformer coloration, which classically speaking a lundahl isnt going to give you. Jensen makes plenty of transformers that are "clean" sounding, yet image completely differently than a lundahl. You seem hung up on specs which you should really just forget about, I have yet to get to a point where specs translate to any kind of sound that comes out of a speaker. You really need to listen to this stuff to really know. Probably not the advice you are looking for but maybe a nudge in the right direction. Start listening to boxes out there and making notes of the transformers in them. Until you sit with a circuit and plug transformers in and out you are never going to definitively know what is really happening with them.
dave