"I’m looking for a "neutral" preamp that does not emphasize and de-emphasize any frequency range but has just a tiny bit of warmth."
The Benchmark LA4 preamp is my fav but it has no warmth. I love it with my KRELL Dou amp, which is warm. It also is great with a CODA #8 amp, which is slightly warm.
You will not find anything more neutral than that, expect the Topping pre90 that is very similar to the LA4 and much cheaper. The pre90 does not mate well with all amps, while the LA4 is bullet proof. Do not let the low price of the LA4 fool you into thinking it is low quality. I think it is the best preamp out there. I like it totally neutral.
I also own a CODA 07x preamp and it is colored but also a great sounding unit.