If the lease does not prohibit you from playing

instruments, you are free to play it as long as your practice time is reasonable, normally from 8:00am -9:00pm. What you can do is to write a letter to the management company citing the relevant lease terms. This is to tell them that it is within your rights to play plano around before 9:00pm. And then you should find yourself a lawyer to write a simple letter to the tenants downstairs to tell them not to interfere with your life any more. In the letter, you should make it clear to them that you will go to the court if they keep on bothering you by knocking your floor. Such a letter won't cost you more than $200 if you can find a competent Chinese lawyer.