First, I would like to thank everyone for the positive comments regarding my Trumpet MC. It's a design I am very proud of, being able to reduce the cost of a world-class differential tube phono circuit design down to a price regular folk can afford.
As for tubes, I find the X7 tend to focus on tonality (plus noise and microphonics in the first stage), whereas the U7 delivers dynamics, weight, and body to the music.
The 5814 may work ok in the U7 spot, even with the slightly increased heater demand, but not a 12BH7.