Hi @mattw73 , no worries happy to provide an update.
I didn't end up pursuing my findings with Schiit, though I continue to be a happy camper using this preamp and would encourage you to pick one up if you can still get one, especially with the balanced options. It's a steal.
I learned shortly after my last post that my microphone was no longer compatible with my recording software so I put a pause on further recording and have just been enjoying the music.
However, I have a UMIK-1 in the mail right this second as I was interested in exploring convolution filters within Roon and HQPlayer so I might pick this back up and can let you know what I find.
Regarding my experiences in the subjective realm, I've tried out some additional NOS combinations, and I feel there are differences, but they are definitely subtle. Most notably, there seems to be less energy and dynamics whenever you deviate too much from the recommended 2.4A heater current total of the four tubes. Using 4x 6922s just feels too mellow for instance, but adding 2x 6N6Ps to the output side restores the excitement while letting some of the character of the 6922s come through, which with my Bugle Boys, I interpret to be a tiny bit of warmth, with a slightly wider and more holographic soundstage compared to the SS Buffer. As tempted as I am to spend $300+ on a pair of NOS Mullards, I just feel that this application wouldn't do them justice. Luckily 6N6Ps are plentiful and cheap though.
So in short, I think you're right to call this an entry point to tubes, but it's still worth it in my book given the range of features and strength of the solid state modes.