As a followup, I finally got a chance to run these at a classical choral concert with orchestra, and while I don't normally post concert recordings {1}, I'll say that I'm extremely happy with how the X-X set sounds.
They're a bit on the hot side - with the Littlebox's input transformers engaged, I can turn the preamp pots way down, and even then I have to turn down the level control on the LS-10. If I were recording a loud show, I'd definitely need to turn off the trafos.
The X-X > Littlebox > LS-10 line-in setup is definitely quieter than running my SP-TFB-2's directly into the LS-10. How much of that is the mics and how much is the preamp is hard to judge, but the X-X/Littlebox combo is a winner. While it isn't as
as SP-TFB-2 > LS-10, the better sound is worth the effort.
{1} A personal decision based on my sense of fairness to the artists, but that's off-topic.