That was probably me! I notice at least one more has sold since I purchased mine just after Christmas.
I have used the M2 with the early firmware it shipped with, then the 2.56 firmware version and now the last 2.63 version as downloaded from the Acoustic Research web site. To my ears and using the Sennheiser HD 58x headphones, I find that the sound quality from the original AR Music Player is different than that produced from the newer AR Music Player PRO app. The original app seems to have a slightly deeper, fuller, more pronounced bass and maybe smoother, yet more lively mid range, which I prefer, when directly compared to the AR Music Player PRO app. Unfortunately, the original app is not as intuitive or easy to use as the USB Audio Player Pro based app but since I bought the M2 for ultimate sound quality, I find myself always using the older AR Music app. I don't know if this difference is simply down to default EQ sound choices made by the app developers or if its due to some technical difference in how each app utilizes the hardware.
Thanks for this as I'm close to have the same feeling about sound quality from the two different apps. I like the new features offered by the Pro version but I guess there is something that could explain the differences clearly heard when comparing both apps. My idea is that the way the two apps are integrated is different: maybe the first one is bypassing some Android system layers when the Pro version is closer to a standard Android app and uses system services that could affect the sound quality. In other words, the original version comes closer to the audio hardware than the new one. It could interesting to hear AR voice on this topic.
Enjoy the music with the AR M2.