I did some reading... and I understand now. It is the Ion that used the AL3101 voice chip, and that is the one available from Wavefront, and the ones that I have. Of course, the A6 uses an analogue voice chip, like the Roland Juno 106, etc. If that IC is really unobtainium, it might be possible to reverse-engineer the IC, and re-engineer a 'replacement' - probably a small PC board that you solder in place of the original voice chip. That is what somebody has done with the Roland Juno 106 voice modules.
Doing this would require a skilled reverse engineer to have a synth with at least one working voice chip, a 'scope, possibly a logic analyzer, and the service manual, and a whole lot of time. The other 'option' which Alesis could possibly do is have a firmware update that allows you to selectively disable voice chips so that you'd not have a dead note appear once in a while - the synth would just behave like a 15-voice synth instead of 16-voice. It's 'only software'.