Thank you for the kind words. Congratulations on the RP-1.
My experience with the NOS Mullards that I got from Kevin at Upscale ended up with an ear ache while they were transitioning to sweet sounding tubes. I got hiss from one channel and did not hear it until my ears hurt. Oddly, it was only on certain tracks for whatever reason. Upscale states on the box that they were burned in for 50 hours but expect at least the first 50 hours to be one where you should either listen to near mute levels or not at all. I had not experienced this type of fits of hiss and distortion from a tube before.
I should also relay to you that during this time I switched out my silver interconnects (DAC to Preamp) to copper ones and the sound was immediately much better. May switch to silver again at some point but for now I feel no need to go back.
The Mullards are the first valves that I have "heard" and I am smitten. The RP-1 is generally neutral, but the Mullards are not, and thus, the journey continues..
Let us know how it all works out with your new gear.