I figured out what was going on and I'm slightly frustrated by it, as the configuration, documentation and hardware implementation by both Schiit and Apple is poorly implemented and explained. It turns out that Apple requires a lighting to USB3 dongle to connect iOS devices to USB C inputs. I had to do a fair amount of digging to discover this, as it is not particularly well detailed by either Schiit or Apple. I'm not thrilled to have another device in the chain and frustrated that I couldn't just use a Lightning to USB C cable, but we'll lay most most of that on Apple, but given the proliferation of iOS devices and the growing market share of Apple Music as a streaming service, Schiit ought to feature that detail more prominently and have a better documented FAQ and trouble shooting section.
Another issue I had related to their use of USB C for a power input cable. It's neither clear nor intuitive which serves as an input and which does double duty and I was flummoxed by its implementation. Using a standard center negative power plug would have made this a non-issue.
Finally, I hooked up my Oppo BDP-83 via a TOSLINK SPDI/F cable. The disc inside it was Beck's Sea Change, in SACD format. What I hadn't realized was that thanks to Sony's overly restrictive licensing, digital SACD info can't be transmitted via SPDI/F cables. Since I'm new to the format- I digitized all of my media 16+ years ago and never had an SACD or Blu-Ray player, much less a transport and DAC, I own my ignorance on the matter, but also fu Sony. That was solved using an HDMI/Audio splitter that I happened to have on hand, running the TOSLINK from the splitter to the MODI+. Again, I'm somewhat frustrated to have another device in the chain, but it works.
Initial impressions are good enough. I've only listened to it in my spare bedroom recording studio, via my Glow Audio Amp Two EL84 valve amp, through a pair of somewhat ancient KRK K-Rok near field monitors. At 3 feet apart and 3 feet away, imaging and staging was excellent; I only experienced one jittering hiccup, that was probably related to my iPad going to sleep, than the DAC, but I haven't yet replicated it. Next step is to run it into my Marantz AVR and Super Heresy speakers and compare and contrast it to the Burr-Brown DAC, there. I'll be able to get a much better impression there, as the superior speakers are less flat than the monitors, the listening position, speaker spacing and toe-in are better and I'm generally more familiar with the other gear involved.
Also, kudos to Schiit for their prompt responses, although I regret that contacting them was even necessary. Improved documentation on their website and manuals, not to mention a better, more intuitive input interface, would likely have rendered this ordeal unnecessary.