Zuki, relax and take a deep breath...
I have never heard a "bad" sounding Solton /Ketron product..going back to the mid 80's...

Something is definitely wrong here....Do the factory reset..( should have been the first thing you did after opening the box)..

Maybe somebody is really toying with you..and has things messed up..

There are a lot of things with Ketron keyboards that I do not care for..but, sounds are not one of them..

Let us know if the factory reset helped..If not we will pray for you..