HT-6000 is the high-end version of CASIO HZ/HT series,
the Spectrum Dynamic (SD) synths line
(see also CASIO MT-600 page for sound examples of SD synthesis).
It was released in late 1987 and lasted until 1991.
CASIO advertised that velocity and 'Line edit' were added,
but, again, virtually no further details were known.
CASIO HT-6000 (1987)
Recently I got an HT-6000, and to my delight I found that it has a remarkably advanced synthesis engine.
While HT-3000 can be seen as an advanced Poly-800, HT-6000 is, to me, one of the greatest DCO analog synthesizer.
Overview
HT-6000 is a 4DCO analog synth with a ring modulator.
VCF became polyphonic, and white/metallic noise can be controlled by an independent DCA.
For details and sound examples (recorded without external effects), follow the link.