I am also on a very narrow path to find the right tube preamplifier for my class D monos from Merrill Audio that I just purchased a couple of months ago. The funny thing is every person I talk to gives a different response or direction to go, but I guess that is part of the journey that makes final result so worthwhile. I have tried to narrow the list down to as few as possible in the 4k used price range, here is what I came up with.
Backert Labs Rhumba Extreme
Zesto Audio Leto 1.5
The RP-7 seems to be the best bang for the buck and most versatile for inputs/outputs functionality. Love the looks of the Zesto Leto 1.5 it is a piece of art, but also the most expensive. I know the Backert Labs Rhumba Extreme works well with the Merrills because the former owner also used one. I also need HT bypass which all of these fine preamps have. Little surprised that nobody has mentioned the Zesto Leto yet or even an Allinic L 1500. Happy hunting.