So, I fixed on the Lumin U1 Mini, which just arrived and was installed, replacing the Node 2i. And the whole system is back up after repairing/recapping the preamp.
The difference is NOT subtle. I feared that it would be, or maybe not much difference at all. The U1 is a transformation. Most listening has been CD-quality upsampled to 24b/88khz (which seems to be a little better than the native stream). WOW. The improvements are mostly extremely low level detail - vocal inhalations, cymbal details, piano high register notes, that kind of stuff. The Maggie ribbons are extremely revealing and seem to be getting what they deserve. Bass also benefited. More controlled, better resolved.
In short, the U1 Mini uncovered things I simply hadn't heard before. I was happy enough with the Node 2i, just felt that there was room for improvement. I didn't know how much room! $$$ well spent. Now I have to re-listen to everything.