Another thing I tried to accomplish is chords - polyphony, currently it's not possible and I found this on the tone.js page:
All instruments are monophonic (one voice) but can be made polyphonic when the constructor is passed in as the second argument to Tone.PolySynth.
//a 4 voice Synth
var polySynth = new Tone.PolySynth(4, Tone.Synth).toMaster();
//play a chord
polySynth.triggerAttackRelease(["C4", "E4", "G4", "B4"], "2n");
The question is, how to pass an FM-synth to PolySynth, so that you can play chords on FM-synth