Good morning and thanks for the feedback. Yesterday was a tough 14, hour day. My apologies for not being clear. Comments inline below...
I'm a little bit confused--not sure exactly what you mean by "get the Casio midi to the Audiobox phones."
We know that MIDI is a protocol for sending control information, not audio signals. So I take it you are using the PX-850's MIDI output to control a software piano on your computer? And you want to listen to the resulting sounds through the 44VSL's headphone output, correct?
Then you basically need 2 USB connections. The first USB cable (from DP to computer) sends MIDI control information to the software piano. The second USB cable (from computer to 44VSL) sends digital audio to the 44VSL, which performs the D/A conversion. You turn the 44VSL's Mixer knob all the way CW to the VSL position, and adjust its Phones volume control to taste.
This is my exact configuration.
As long as the software piano is configured to recognize the Casio as a valid MIDI input device and your computer is configured with the 44VSL as its selected USB audio output device, you should be golden.
I tried using Sibelius First, Home Concert Extreme, and Synthesia. None generate any sound in the headphones connected to the 44VSL.
Since the sound works fine when the Casio is hooked up using the Casio Line Out (or headphone out) to the 44VSL's line in, the 44VSL's preamp seems to be working fine.
You can leave the Casio's line outs disconnected unless you want to listen to its internal sounds, in which case you should temporarily disconnect the Casio's USB/MIDI cable to keep from creating a ground loop. Also, remember to turn the 44VSL's Mixer control back to Inputs when listening to the Casio's line outs.
I tried this and it appears to work fine.
If you want to leave the audio cables connected all the time, you can install a humbucker (like the Ebtech HE-2) or a DI box (like the Radial ProD2) in the signal path between the Casio's line-outs and the 44VSL's line-ins. Either of those devices will kill the 60 Hz ground loop by using passive transformers to buffer the 44VSL's analog inputs from the Casio's path to ground.
I'd like to get MIDI-only working, but it would be great to have the flexibility. Assuming the sound is the same, I'll probably get the Ebtech HE-2.
Does this all make sense, or am I missing something?
Definitely makes sense. It looks like I'm missing something subtle with the MIDI connection or signal path.