which preamp sounds most transparent?

Try an autoformer based pre. This may be the most transparent, but may not give you the drive it turns out you like.

H-Cat 12B with Wavefront Timing Control. Incredible preamplifier.

I am using no-preamp right now (Accuphas cdp directly to CJ amps) - don´t missing my Premier10 a bit.

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.