PrimaLuna doesn't seem to get as much love around here compared to many other brands. Not sure why that is. I tried almost a dozen integrated amps (mostly tube based) not too long ago and ended up buying the Evo 300. It just sounded "right" in my setup, not too warm or bloomy, not to bright or etched. I love it.
The 400 didn't impress me as much. Not only did I not require the balanced aspect, I actually didn't think it sounded as good anyway.
You definitely would want to try tube upgrades once you adjust to the base performance of the Evo 300. There is a lot more SQ hiding in there that better tubes will unlock.