Neutral preamps

You might want to listen to the Hovland. It has changed my feeling about tube preamps. I have owned many tube pres and they for the most part have that not unpleasant bump in the middle. Warm, dark, rich whatever. The problem is that even slight adjustments are now not acceptable given good source and good components on the back end. The Hovland does not make Ella seem like she is sitting on your lap or that her head is six feet wide. I found the Hovland to provide much more detail from my source and for good or bad I think you need to hear what the producers intended.

The BAT VK-30 SE and BAT VK-50 SE sound very neutral to me.

I heard it against the Aesthetix last year. Prior to that, the Asthetix was the best I'd heard.

I don't know about the most neutral but, I think the Sonic Fontiers Line 3 is amongst the most neutral tube pre-amps.

If you haven't please try any of the levinson's (38, 38S, 380, or 380S). I have factory upgraded 38S to 380S status and it is smooth and quite and fast. It works well with every amp I've tried with it. Give it a try.