The difference between powering the Stepdance at 12-Volt and 15-Volt is subtle with some recordings but undeniable with others (at least with my source - Sony PCM-M10, my headphones - LCD-2, and my ears.)
I wouldn't begin to claim that I am a skilled listener - not at all like many people here in these forums who can describe nuances that I'm sure I can't even perceive. But I'm getting there, thanks to my relatively recent advancement to using nice (well... nicer) hardware.
At the risk of embarassing myself with my limited vocabulary of acoustic descriptors, I will attempt to describe the difference between what I hear with 12-Volts vs. 15-Volts (or 8.4-Volts and 15-Volts, for that matter) - again, using the Sony PCM-M10 and Audeze LCD-2. Don't laugh!
Recordings that have a lot of texture sound tighter, more precise, cleaner, more distinct. If I were to use photographic terminology, I'd say that the sound has higher acutance and resolution, where acutance is edge sharpness and resolution is a measure of detail. Everything is more in-focus, less diffracted. A recording of a sustained note from a flute wouldn't exhibit much of a difference in terms of detail, but recordings that have an electronic buzzing sound, for example, where the notes, if put to paper, would have to be described as 1/64th notes or even faster, are greatly improved by an increase in supply voltage to the Stepdance. These really fast on-again-off-again sounds are much more crisply presented - there's just less smearing of the boundaries between when each note starts and ends.
Hans Zimmer's soundtrack for the movie
Inception has many such sections that are rich with detail that just become all the more finely presented when the Stepdance is outputting higher voltages (more watts) to the LCD-2s.
But an even more pronunced improvement (again for my ears, my hardware), is the increase in dynamic range that can be heard with recordings like
Rickover's Dream from Michael Hedges album,
Aerial Boundaries. There are instants of startling impact, where the forceful plucking of guitar strings are already stunning with the Stepdance operating at 8.4-Volts (internal LiPo battery), but when I switch to 15-Volts, these moments that call for a lot of power go from being a dog that barks to a dog that bites. The signal is just unleashed - presented without constraint, with what are surely accuracies of signal amplitude in addition to accuracies of frequency, that just aren't available with fewer watts.
I genuinely understand now, why it's said that having more watts at your disposal isn't about playing at higher volumes. I can only imagine what a truly powerful (15-Watts RMS per channel) amp like the Leben CS-300SX (or whatever it's called) must sound like.
I'm not going to go anywhere near trying to describe soundstage and imaging differences, if any, because frankly, I'm not sure I even understand those terms yet, much less how to make comparisons of such attributes.
Ugh - I'm such an engineer. I hate writing about abstracts like this. I'm going to hit the Submit button and try not to look back.
Mike
I copy and pasted the link after I searched the thing for you but it didn't appear. I guess it's because I should've used the code for it. url=blahblah. But yeah, glad someone else helped ^^
I'm letting go of my fullsized rig. 
