This is my 2nd attempt at synthesizing a piano, and I think it sounds quite a bit better than my first. I did limit the notes that can be played, especially the higher ones, since I still haven't figured out how to make convincing high-pitch piano notes.

I accomplished this sound by separating the octaves of the piano into many different part numbers inside ZynAddSubFX. The part number is basically a new layer that can be applied to a synth. I also limited every part number to have a range of 1 octave, so that I can change how the piano sounds one octave at a time. I did something that I like to call "interlaced octaves", where I made each octave of the piano overlap each other, so that the piano smoothly transitions between the different part numbers with different changes to the oscillators.

This means that there isn't just one oscillator that will change the sound of the entire instrument, but several different oscillators that alter the sound. So far I have 8 part numbers, which is difficult to work with, because each part number is independent of each other. So, if I wanted to change how the piano sounded, I would need to change ALL of the part numbers.

I'd like to find a solution at some point that is far less complicated, but this is the best I have for now. I really think it's possible to make a fully-convincing synth piano using ZynAddSubFX, because I've gotten this close, but also because of how powerful it is. Just by using ADSynth alone, that part of ZynAddSubFX enables you to add 8 voices to the synth, and combine them / subtract them from each other. And then because ZynAddSubFX has 16 part numbers, that means you can make a synth with 128 LAYERS of varying oscillators that combines into one synth. I think that is enough oscillators to replicate a real piano.