Thanks, nickd. I got interested in this preamp from a thread on the WBF. Per below, does this poster's description of the sound resonate with you? (The poster is referring to his amps but it applies to NAT's preamp sound as well.)
Back to the Nats. Despite the steep learning curve I'm having, they do something that I only hear in pricier gear like the $25k Koda K10 preamp, $15k Connoisseur line stage and $100k+ Ypsilon SET100 monos. And that is total tonal saturation thru the mids into the bass which doesn't cross into euphonic cloying warmth, so bridges the gap to how instruments and voices sound live, making the listening experience more like the real thing, more immersive, and I believe has a positive effect on subconscious awareness of timing cues.
Much as I loved my Hovlands and Audions, I got nowhere near this expression of colour and density w/them. A total triumph, and at price levels 3-5x cheaper than SOTA offerings which I find hard to believe are vastly superior (I had a long stint w/the $25k Koda and wouldn't swap the Nats for it), it's a brand that a lot more discerning listeners here should seriously consider.
The Audion Black Shadows, seductive as they are, just can't compete with the Nats' tonal richness that just makes the mids so alive and colourful. And critically, unlike a lot of tonally warm tube amps like BAT that are full in the mids but wooly and smeared in the bass, there is no compromise in the lower frequencies. No gauziness at all. Like the best of a glass of full bodied red wine, and the crispness of a glass of white. Audion is pretty good w/tonal color, but the density/speed balance of the Nats just races away.
What is interesting is that the Nats dark balance that I was unsure about two years ago I have interpreted as a real culture shock change from sparse, skeletal Hovland house sound to a dense technicolor presentation that I was just not ready for, and rejected as too alien. But the stepping stone of Audions to the Nats is a better transition, and now this tone dense sound I can't get enough of.