which preamp sounds most transparent?

John is right. Others will have differing points of view. It seems that you are looking for a neutral pre-amp. I have been taken to task over this before since some people regard neutral as synonymous with flawed, but I think Klyne pre-amps are very neutral. Or in other words more 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.