I have the PSR-E433, essentially an older version of your keyboard, but with five octaves instead of six. To get a more enhanced piano sound, here are the steps you can try. You can try each one, in succession, until you find a piano sound that you like.
First, I would add some reverb and chorusing. I usually use one of the Hall reverbs, and for chorus, you can try the different versions, but for this purpose, I would stay away from the flangers. I would put both the reverb and the chorus levels up about half way, to a level of about 64, but you can try different levels to suit your taste.
If you want a fuller sound, you can add another live piano sound together with the original using the dual voice, but set this dual voice to one octave lower than the main voice. Keep the reverb and chorus on. And try different volume settings between the main and dual voice to see what sounds better to you. Adding the reinforcing sound one octave lower can give the piano a deeper, more substantial sound.
Finally, you can try switching the dual voice to one of the other piano sounds, like bright piano, to see if you like that. However, on my keyboard, I've kind of grown tired of the bright piano voice, but still, you can try it in moderation.
And of course, you can try subtle changes to the filter cut-off and resonance -- but if adjusting this at all, I would only make minor changes, because anything more, can greatly alter the sound so that it no longer even sounds like a piano. That is great for synthesizer sounds and special sound effects, but not so great if you're trying to get a good acoustic piano sound.
And of course, these are all just suggestions. These keyboards have a great amount of ways to tweak and customize the sound, so just experiment and see what you come up with. If you like a sound, you can save it to a registration. If you don't like a sound, you can start over and try again.
