Thank you for your interest in the MX49. We think that the MX49 is possibly the best value synthesizer available today - it's inherited both its synthesis engine and its waveforms from our Motif Series which - especially for piano sounds - has became one of the most popular synthesizers on stages around the world.
The piano sounds included in the MX49 are very good - and they are stereo sounds, designed to be heard in stereo (either with headphones, nearfield studio monitors or stage PA speakers*). If you're in a situation where there's no way you can plug into a stereo setup (perhaps the PA is only mono for instance), then you'd put the MX49 into Mono mode - by only using the 'L/MONO' socket and by not having anything plugged into the 'R' socket. Mono mode should sound fine on stage, but won't sound as brilliant as using it in Stereo mode (which sounds fantastic).
The worst possible way to connect it would be to have both 'L/MONO' and 'R' outputs connected to speakers (perhaps via a mixer or power amp) which are working in Mono. In this scenario, the left component of the sound is merged with the right component - resulting in phase problems, cancellation of frequencies and all sorts of horrible consequences from a sound point of view. It won't sound anything like how we sampled or designed the sounds.
Regarding the built-in demo songs, all the sounds you hear are being 'played' live by the MX49 (as if a computer or iPad were sending MIDI data in realtime) so those sounds are available. Compared to the demo videos, certainly any produced by Yamaha will also be using the built-in sounds though it's possible additional EQ was added after recording (this is purely speculation) - but nothing that you wouldn't be able to acheive from the MX49 itself.
If we had any advice, we might suggest going to the slightly longer MX61 if you'll be performing piano accompaniment (or to look at our powerful MOXF Series if budget allows). But otherwise, the MX49 is a great synth.
*If you're plugging into a stereo mixer or PA using two mono channels, ensure you pan the 'L' channel hard left and the 'R' channel hard right