It may be that you have a MIDI problem. As I understand it, you can load the files into Vsig without problems – this suggests that your MIDI Receive and the database are good. The problem would seem to appear the other way. My guess is that for some unknown reason the data is being corrupted when it is sent back to the H8000.
Best way to test this is to use another MIDI interface or serial.But, you are having trouble with serial. Make sure that its settings (especially baud rate) match the H8000.
You should see the BUSY light by the card slot flash when you send it anything, whether or not it understands it – if you do not, you have a computer hardware issue.
At this point I am unconvinced that the problem lies with the H8000. But of course, I would say that ..