No question that talented/skilled musicians can pick apart songs and it can be helpful and useful in headphone evaluations. At the same time, my point stands that we can still be bias based on our experience and preferences.
Take your wife for example. You say she likes the PX8, and you used her credentials as a musician and singer to validate her preferring the PX8 to the Bathys, an overall more analytical, revealing headphone. The only logical reason in my mind that a musician or singer that values reference sound can prefer a less analytical headphone, the ultimate in hearing clearly all of the sounds, is if they have some sort of bias to the way the more colored headphone is presenting the music to them. Unless you only prefer reference headphones, you have to have some bias. At least that's how I view it.
I myself have some bias and I'm open and honest about it. It doesn't mean that I can't recognize headphones that are more revealing and accurate, but I tend to grab my more colored headphones most. As of late, my M&D MW75 has been getting the most play from my collection. Why? It has deep punchy bass, slightly accented highs, and a wonderful tone similar to the 5909. Drums and bass sound excellent through them. Drums have weight and the cymbals are tonally exciting and aren't as much in the background like they are through the PX8. Both the PX8 and MW75 are what I consider fun headphones, but as a drummer, I prefer the MW75. If I was a piano or horn player, I may prefer the PX8 because of its more highlighted, and musical midrange response.