Bernard,

The SD4 is like having 2 SD2s, and I used one side with a Godin MIDI guitar connected to a Roland GI-20 (no longer made) and then to the SD4. The other side was connected to Realband. Nice thing was to be able to change the MIDI guitar patches directly from the buttons on the SD4. I didn't really notice much of a sound quality difference between the SD2 and the SD4. I'm told the SD1000 is an "upgrade" to the SD2, and I'll have to check that out when I receive it.
- Rick