Daren, I owe you a comparison for my claim of the Yamaha P37D being flat on loud playing.
Therefore I performed a test where I played roughly the same melody on 3 different octaves.
I recorded all three octaves on a row playing a somehow similar level, using a gain which matches the level of playing.
Then I interleaved them together so you can hear the line played softly and then loud, each time on a different octave. I also normalized each line so they sound roughly the same level, so you can get most of the sound difference without the misleading effect of different volumes.