> channels 1, 11-16 do not work.
I get the impression that your E-15 is operating in its normal mode because the first MIDI implementation chart in the manual says:
* Channel 1 defaults to "note to arranger" (which sounds like it would be used to set the accompaniment chords) and "basic MIDI channel" (which is used to send mode messages), both of which sound like they might not actually make any sounds.
* Channel 11 defaults to the "sound effect" channel. The manual doesn't say what that does, but as a guess, maybe it only makes sounds on 6 keys that represent the 6 sound effects buttons on the panel? If so, then it probably doesn't make much sound either.
* Channels 12 to 16 default to no assignment.
I got the impression from the second MIDI implementation chart in the manual that the GS mode would allow all 16 channels to be used.
If holding down the MIDI SELECT button at the same time as you turn on your E-15 didn't help, then maybe sending the System Exclusive message to turn on the GS mode will work better for you:
F0 41 10 42 12 40 00 7F 00 41 F7
As a test, the attached MIDI file contains that System Exclusive message followed by a different instrument note on each of the 16 channels.
Download: 16.zip