Look guys... doesn't ANYONE have a simple audio recorder external to the S900 or Tyros?
Put up a short MIDI file. Preferably a piano piece, using the stereo piano. Stereo drums, some kind of sound with chorusing on it too (maybe a nice pad?).
Record it once in stereo, record it once in mono using the mono out jack.
Send the .wav files (NOT MP3 conversions) to me (use Yousendit if bigger than 10-15MB email attachments) and I will do the work comparing properly collapsed mono derived from the stereo with the mono out jack. I will post the results of the comparison.
DON'T play on the MIDI files. I want both recordings exactly the same.
Let's put this issue to bed once and for all...