Personally, I don't think anyone has ever bothered to try to quantify the mythic 'mono out isn't as good as summing L&R externally'.

It's fairly easy to test if you have a DAW. Same Piano MIDI file (why not use the Purgatory Creek one? ), first few bars. Make one recording stereo, make another recording mono, out the 'Mono Out' jack. Convert the stereo recording to Mono in your DAW, then 'normalize' both files (this is important, to make sure they are EXACTLY the same volume).

Now compare first by listening, and also if you can line them up to play simultaneously (line the peaks up to be sample accurate with each other) flip the phase (Invert phase) of one of the recordings, and then play. If they are IDENTICAL, one will cancel out the other, and you should hear basically nothing at all.

If you can hear a difference, it should show up on the null test, too. But unless there is something seriously out of whack with the summing electronics in the arranger, you should hear very little.

Personally, I would be surprised if there WAS much of a difference. Summing two signals to mono is not exactly difficult, electronically. But gain staging TWO outputs (L&R) into two channels panned mono, and ONE mono output into one channel so they are at EXACTLY the same level is quite hard. And the ear perceives even the slightest change in volume as a 'difference' in tone. Hence the 'anecdotal' reports of a possibly flawed Mono out.

Me, I think the problem is MUCH more about certain stereo piano sample sets (to my ears, Yamaha are particularly bad in this regard) being somewhat out of phase (to exaggerate the apparent width) or poorly recorded, so that there are really noticeable changes in timbre when collapsed to mono, no matter HOW you achieve that...

Making your manufacturer aware of your displeasure at how poorly the stereo sounds collapse to mono will do FAR more than worrying too much about going mono 'inside the box' or outside. They either take it into consideration when they make the NEXT arranger, or it will never improve, no matter WHAT the do to the Mono jack...

)BTW, I have done simple tests on my G70, and have noticed NO difference between going mono inside the box or externally)