Apart from the CX5M and CX5M-II I also own a TX81Z. Why? Do you ask – I just say: “LatelyBass”!
In more words, this TX81z is a 4-Operator FM synth like the CX5M but you have the ability to change the operator waveforms itself. This allows you to make more complex waveforms with just one operator.
Matt Gregory has a very informative webpage for the TX81Z which also explains a lot about FM-Synthesis and programming; also a good read for the CX5M users: http://the-all.org/tx81z/ Anyone who also owns the TX81Z by the way is more or less obliged to have this editor.