Rebirth even ran adequately on my old 6100/66Mhz 601, so it should be no problem on a 6400. In fact there might be more capable and elaborate synth programs out there if someone's written one tight enough to run on that hardware. That's a big if, and I can't off the top of my head suggest one. I would go have a look at hitsquad.com's Shareware Music Machine and see what you can dig up.