Thank your for you question. I’m also a Line 6 Helix user, and yes, it possible to control the H9 via the Helix Floor using MIDI. You can even open and close effects loops at will with snapshots if you want.
In the MIDI world, two or more devices need to be on the same MIDI channel to communicate. Usually the default is either MIDI channel #1 or OMNI, meaning the pedal will receive commands from all channels. As long as your Helix and H9 are set to the same channel that’s what’s most important. Take a cable from the Helix’s MIDI OUT to the MIDI IN of the H9 and try going through the presets on your Helix. Does the H9 react sequentially as you change presets on Helix? From factory, I believe this should work automatically. If not, remember, make sure Helix and H9 are on the same MIDI channel.
Next thing to work on is inserting an FX Loop block in the Helix. Are you running your rig in mono or stereo, because there are FX Loop blocks that correspond to both. You also have to decide where in the signal chain you want the H9 to appear – after the preamp stage but before the cab, after both preamp and cab, or even straight into the amp if you’re running things clean. You can find videos on how to do this on YouTube. Here are a few:
Helix allows you to inject your H9 using FX Loop blocks or individual FX send and FX return block, in case you want to send the signal to the H9 after the preamp but return it post cab or whatever you want to use. Experiment to find what sounds good to you.
This is a good BEGINNERS GUIDE TO MIDI: https://youtu.be/JIzCEYvwt1M
Lastly, you’ll want to organize the presets on H9 so they work with the signal chains you’ve created on your Helix. This is easiest to do on H9 Control. Create a preset list, organize them in the order you want, then download that list to your pedal so that your preset changes occur in order.
I think that’s enough info for now. Feel free to ask any questions as you go.