Another small clue. From the desktop PC (that's the one I'm having trouble with), when I hit various buttons on AC-7, I get a MIDI signals showing up on channel 1 of the rtpMIDI Sonar input as notes in track view. This behavior does not show up when using the laptop (the one that works fine with AC-7). Could it be, that the desktop Sonar believes that AC-7 is a normal instrument and keeps the control surface connection request from getting to it? Still digging...Stay tuned.