Hi Clio09,
The post about Stereophile's top award to a USB dac was mentioned to demonstrate that we are not alone in our findings about USB's potential and mention the fact that other serious audiophiles have also found that some particular USB connected solutions can sound amazing. BTW: Were you aware that the DAC that Stereophile awarded uses Gordon Rankin's technology?
As far as the "proof" you want, we will stick by our personal mantra ; The superiority of ANY dac design should actually be "measured" by it's end SONIC GREATNESS or lack thereof, period. Anything other sort of analysis-methodology, other than how the product actually sounds, is just conjecture and speculation that holds very little merit for a true music lover. If the DAC design sounds as good or even better than some of the best digital in the world, cost no object designs included, then the "proof" has been met.
Per your question about the Music Streamer Pro and m2Tech hiface, we think that both are great "budget" offerings at their respective price points. Comparatively, the Tranquility DAC was NOT designed as a "budget" offering that was just built with "off the shelf parts" or budget connectors or budget power supplies or with budget level output stages.
In comparison, the Tranquility has MANY proprietary circuit refinements that took years of research and development leading to proprietary optimizations incorporated within. We are proud to say we've found optimizations that no one has ever even considered in any DAC, at any price! Top level passive components, capacitors, custom designed power supplies and regulation, custom discrete output stages along with all of the proprietary optimizations to boot! All of this was done in a quest to create a digital solution that offers a whole new level of sonic greatness.