I’ve owned three of these over the years – not just for the Lately Bass patch, they’re a really interesting step in the Yamaha FM chronology. Unlike the DX7 which only used digital representations of sine waves, the TX81Z allows you to use different waveforms in the generation of FM patches, which really expands the palette of sounds you can make.
Hard to see it go, but I’ve been using my MODX these days for FM so up for sale it goes.
More details:
http://www.vintagesynth.com/yamaha/tx81z.php