I think I located the weak link in the chain (from JDS’ website):
"* Make sure you are using a genuine Lightning adapter. The following model is supported:
JDS Labs has confirmed DAC support for iOS versions 7.x through 11.x (most recent build as of January 2017). Customers have reported DAC connectivity success with iOS 7.x, 8.x, 9.x, 10.x, and 11.x. Please note that support is dependent on the mobile operating system and can vary with each operating system update!
The older Lightning to USB Adapter, part #MD821AM, may be used with iOS 7.x thru 10.x, but does not support USB Audio when used with iOS 11."
Damn you Apple!