I emailed Kingwa and he gave me a pretty detailed response that I think is worth sharing here:
(Just changed a few grammar-related things here and there).
The Master 7 has a different sound from the Reference 7.1. We picked up on the advance of the ES9018, the qualities of the PCM1704, and redesigned the new DSP software to process data. The M7 overall is better than the ES9018 DAC while it retains the neutral sound of the PCM1704.
I can't be sure if you would like the sound. It's much more transparent and dynamic than the Reference 7.1, but still has a rich (not dry or thin sound). But again, a very different sound.
The Master 7's upgrades over the Ref7.1 are not just limited to the USB input, but both BNC and other inputs as well. The new DSP software makes the gear more able to accurately process the data to improve all inputs. However, now the USB32 has very high performance as well, so in most cases, one won't need the extra conversion from a transport as much as the Ref7.1.
I feel the modification of the Ref7.1 to the M7 (if you are interested) is somewhat difficult. This is because it requires replacing a board and do some rewiring. Of concern in particular are the power wires, which, if connected the wrong way, could destroy the board in a few seconds. Maybe you can ask Currawong if it's difficult?
We are willing to offer the upgrade kit for users looking to improve their gear, but because we can't be sure of how well the user installs the upgrade, we can't include any resultant damages under warranty. If something goes wrong, someone less-experienced could ruin their DAC and lose their money.