Audibility is a complex case and I truly believe that you cannot perform a perfect isolated hearing process because the final signal lies in the brain (you can produce sound in your head without hearing them, right ? Maybe it activate your hears but you do not need to have an actual external sound). We speak about electrical interferences but it is somewhat the same for the brain. I am not qualified in this domain at all, but everyone experiences contextual perception in the presence of other stimuli. That is why blind tests are made, but there are still the internal signals like thinking, stress, whatever. That is also why I am in favor of a series of quick A/B since you eliminate much of contextual changes by not allowing your brain to focus on something else.
But I also believe that analytical hearing is a case of brain state of mind and long hearing sessions do relax the brain which can also accomodate and open to more parts of the signal. And maybe it is a matter of dB (because I notice that I generally need to play MP3/AAC at higher levels to have the same satisfaction as FLAC) but in a long hearing sessions I might have more satisfaction/less hearing fatigue with lossless. I will not state anything about this point because I cannot prove it, it is just a thought and experience sharing without knowing about the bias. Maybe small differences which are imposible to isolate in analytical can have an effect of overall perception in the brain.
Bottom line : maybe there are "audible" (consciously or not) differences but at the end of the day wanting no matter what to search for a truth is childish at best and is a way to avoid making choices. We have to accept that hearing is bound to other contextual perceptions and that pleasure is a catalyst of it. Cuisine accepted a long time ago that vision is a part of tasting : take the same ingredients, cooked the same, then the ugly presented dish will taste worse that the perfectly arranged one. The plate, the room, everything count.
Measurements are here to see whether a product has big issues (whether you subjectively like it or not). Then buy the product which pleases you (without making you bankrupt of course) and stick with it. The brain is a powerful equalizing machine : see how you react to TV picture changes. When you first switch - for instance - color setting, you might see a color bias which gradually disappears.
There are too much biases in the hearing signal path to be a perfectionnist in terms of sound. Especially when we speak about DACS.
if you really want to be sure, quick AB things on the same amp with the switch button an "trust" ("accept" is a better word) your findings. Minor differences will likely be eliminated by your brain is there is any. Especially because there is something you cannot do with hearing : compare side by side, and that is why you will never be able to truly tell things apart like you can do with screens.
I you prefer the gustard, return the D30 Pro and be happy with it. Buying something new if you are satisfied with something is a dangerous road and is completly silly.