Bass Notes Out of Tune on Yamaha DGX-650

I've watched videos with people playing the DGX-650 Yamaha keyboard and the notes sound very rich and great. On my keyboard however, the bass notes sound out of tune (don't know how that can happen on a keyboard) and not very rich. Has anyone else had a problem with this?

Also, when I write the music for the notes I am playing it is not written at middle C where I play it, but up and octave or so. How do I fix this?