which preamp sounds most transparent?

I would second Soundlock. I have had passive preamps, tube preamps, ss preamps, autoformers, etc. Never have I achieved the transparency I get with the H-Cat.

I just received a custom built K&K Audio TVC that uses S&B transformers. Initial impressions are that it is very transparent and the ability to control the ICs (which can be a problem with other passive designs) is not an issue.

I would recommend that you consider the Placette Active Line Stage, it replaced a ML-32 reference in my system because of its transparent/dynamic/detailed, yet musical sonic signature.

I ran passive preamps and no preamps for years because I got sick of spending money on active preamps that were no more than fixed position equalisers.
A good active preamp does however give better drive,bass articulation and image focus.Even the cheap NADs do this very well although they colour or degrade the sound in other areas.
When you hear a really good active preamp though you realise that this whole notion that a good preamp should have no sound is a bit of a false god.After all who talks of speakers in this way?And who has a completely neutral listening room?
What is important is that you use a preamp with a sound you like in your system and room,and that sounds good running with a wide range of other components.This does not mean that it needs to be transparent or neutral though.
Some preamps just sound great and in all sorts of systems.This is far more important than neutrality.

Jtgofish - I don't disagree with you. I'm not giving up my active linestage. I like the K&K TVC for digital playback at the moment. I wasn't as enthused about it with analog (vinyl) playback. It was very good, but my Joule preamp was better.