The Buf arrived earlier this afternoon and I wasted no time getting it hooked up!
BF2/64 > BUF > Lokius > Jot 3 - with the BF2 SE out going to the Buf, the Buf out going to the Lokius SE in, and the Lokius XLR out to the Jot 3 XLR in. The BF2, Lokius, & Jot 3 are also connected via balanced XLR to preserve that chain and I just switch the Lokius input to single ended when I want some tubification.
First impressions with the HD800S after a little bit of tube warm up time are that the highs/treble are definitely a little more relaxed. Cymbals and other percussion instruments have what some might perceive as sibilance toned down. Takes the edge off and should be less fatiguing over time. I never found the HD800S highs to ever be problematic, but the difference is there. I will have to give it a go with some other headphones and do some tube rolling after a few others I ordered arrive. Here we go down that rabbit hole again...
Day 2 update: After roughly 24hr of burn in with the stock 6N1P tube and trying some other cans, I'm hearing some more apparent differences. The HD800S still highlights the edge taken off the highs. There is definitely more to the low-mid frequency presence and overall warmth, especially when using my TH900Mk2 Onyx. Using the Lokius input switch makes it easy to do quick A/B comparisons - the SE input comes from the Buf (placed between the BF2/64 and Lokius), while the XLR input comes directly from the BF2/64. When the Buf is switched off (basically turning the Buf into a pass through, bypassing the tube circuitry), I can't detect any significant audible difference between the Lokius SE or XLR inputs.