I think that jnorris's point is that it is very difficult to distinguish between two very good DACs by ear, and it is surprising that someone who can tell the difference between two very good DACs does not find the difference between both of them and a mediocre one significant. But maybe I misunderstood!
Perhaps this source is flawed, but it's debatable that the DACs inside of MBAir, iP6 and iP6S are "mediocre":
Zoomed spectrum of 10,007 Hz 0 dBFS sine wave. (CBS CD-1 track 9, R&S UPL.)
This is FANTASTIC! This is significantly better than previous iPhones, and better than many audiophile DACs, and almost as good as a professional reference DAC like the Benchmark DAC1 HDR.
Spectrum of 1,002.27 Hz 0 dBFS square wave. (CBS CD-1 track 16, R&S UPL.)
This is very good, way better than Bluetooth and better than most outboard DACS.
The iPhone 6 Plus won't distort or do anything crazy when playing modern CDs that are recorded at 110%.