Years ago I had a TX81Z synthesizer unit. It was a rack mount unit (not a keyboard synthesizer; you had to use it with a midi keyboard). It was also an FM synthesizer with a fundamental difference; the DX7 used sine waves modulating each other to produce the sounds, and the TX81Z let you do the frequency modulation with sine waves, or triangle waves, or square waves, or pulse waves. You basically had an order of magnitude more flexibility in creating sounds, and an order of magnitude more difficulty. Some people were able to create some really interesting patches, but I never got good enough to create anything remotely interesting.