Hissing

It is worth noting that there is hiss in all amplifiers and many audio components. The better ones have very little hiss however it is still there. The hiss will be more pronounced if you have nothing connected because the input is open.

Is it a hiss or a hum? A hum could indicate a ground loop and a ground loop isolator might help. Note the use of the word might however. You can get cheap ones for $20.

If your cables are unbalanced there is possibility that you are getting interference into your setup.

Check the hiss does not exist with headphones direct into the keyboard. PC sound cards often have hiss in them you can not get rid of especially if they are cheap and onboard.

The keyboard should be okay but not sure how clean the headphones output is. Yamaha mixers at the low end seem to have reviews stating their USB is noisy and they can be noisy. Your mileage may vary.