This thread is quite long and complex so please forgive me if this was mentioned and I just missed it. Two things:
1. I Don't see a mention of audio connections. What do you have your JV hooked up to for sound?
2. What is a "Supra MD 100" and why are you using it? I tried a google search and nothing came up. That sounds like one of those cheapo Chinese things that sells for something like $5 or so.
You say the X-board shows up as an input device. That means the usb cable handles the midi connection to your pc and all you do is run the midi cable from the X-board to your JV. This is from the X-board manual:

MIDI Connection

The Xboard sends MIDI performance
data to another MIDI
device, such as a MIDI sound
module. The Xboard always
transmits MIDI data except
when “Thru” mode is enabled.

Assuming this problem is not about you forgetting to run audio from your JV into your pc's soundcard and/or an external monitoring system, then dump that Supra whatever it is and run the midi as above.

Bob

Biab/RB latest build, Win 11 Pro, Ryzen 5 5600 G, 512 Gig SSD, 16 Gigs Ram, Steinberg UR22 MkII, Roland Sonic Cell, Kurzweil PC3, Hammond SK1, Korg PA3XPro, Garritan JABB, Hypercanvas, Sampletank 3, more.