Just thought I'd throw it out there: when you say it doesn't seem like a bad solder connection, what is that based on? Visual inspection or actual probing with test equipment?
I only ask because I somehow killed voices 2-4 on my SK-8. I figured the only possibility was that I had spilled solder somewhere and caused a short, but after several days of searching, found nothing. I had only soldered three things to the board, but went ahead and removed them, just in case.
It was only once I went in with my multimeter to start testing for continuity that I found the problem. One of those joints had been heated and reheated too many times and a trace had imperceptibly lifted. Put a jumper in and everything was gravy. My job was easier than yours because it was at least in the analog section of the machine, which meant I could follow signal flow by actually listening, but it may be worth a shot, especially if you have a scope and can get a better idea of what is flowing where.
On the other hand...maybe it is just a scrambled chip..
Saw you moved over to an SK-1 so hopefully this isnt as big of a deal anymore.