I'm looking at Appendix A of the GPO manual, which lists the CC channels and corresponding functions. I see no mention of a channel allocated to send level, and I can certainly relate to your desire to contol that in real time.
I've forwarded your question to the developers at Plogue. If anyone has the answer, it will be them.
"If a man does not keep pace with his companions, perhaps it is because he hears a different drummer. Let him step to the music which he hears, however measured or far away." – Henry Thoreau
Which MIDI CC channel controls the Send level on the Aria Mixer?
The Send knobs in Aria aren't controlled by any MIDI CC.
If you're using a "sequencer," like Sonar or Cubase, I would recommend that you use Aria as the audio engine to get the instruments into your program, but then to do everything else in your software - volume control, panning, reverb etc.
By the way, MIDI controllers aren't "channels." There are 16 MIDI channels available per instance of Aria or other multi-timbral synth. That's the channel specified for individual instruments. But controllers are part of the data which is recorded, and specified to whatever MIDI channel one needs to use.
Well good heavens, of course. I should have known that. In fact I Did, but had let it slip my mind. Don't you hate it when you forget how to do something? I never use the Aria mixer, wanting to use IR reverb in Sonar, so I'm out of practice, but CC91 is the standard MIDI controller for reverb amount, has been since MIDI started. DUR @ me!
A trick I have used before is to create a MIDI file that ramps each CC from 0 to 127 (one CC at a time of course, not all at once!), so that when you play the file in your sequencer you can determine which VST knobs respond to which CC.