No matter what kind of muffler, it will distort the sound. This is caused by the silencer attenuating the vibrations. I personally recommend using rubber ones for a simple reason: they won’t hurt the piano, while metal ones may cause scratches from collisions. The silencer has a good noise reduction effect, which can reduce the sound by 30%-50%, depending on how hard you play the piano. Mufflers are very cheap. Some pianos are delivered directly. If you want to buy them, they only cost a few dollars.