Again, both Gary Diamond and I, along with Hank and Semilive, extensively tested the Yamahas' ability to sum to mono. We all easily agreed, it wasn't done correctly inside the keyboard. Gary can even tell you why, I think.
I just know it's better when you use both outputs.
Regarding the "stereo" ins on the Compact. Yes, the volume is greater when you use both. It sounds fuller. Not certain if that is the entire reason, and don't care. It sounds fuller when you use both.
DonM