Use MIDI In1 and that allows two things - Access to what Ketron calls the GM voices. Now, that is a little misleading. It's more than GM voices. There are actually five banks on SD40 and six banks on SD90 accessible via In1. Bank "A" is actually the GM voice map (like in real General MIDI). The "other" banks allow you access to all of the voices in the unit called GM voices by Ketron. That would be Bank B, C, D, and E on the SD40.
Now, there is an issue if you use In1 - No access to the Arranger section. That is all done via In2. Just be aware of that.
Let me know if that helps or ask more questions if it doesn't.