ricksteruk

Yes it works fine - I think there might be a MIDI thru option you need to turn on somewhere perhaps.
 
But yeah - I've used both my Behringer FCB1010 and also a H&K Midi footswitch with my Helix to control apps on iOS via the CCK.
 
Currently I am trying to set up a rig with MIDI Guitar 2 feeding into Infinite Looper, which then plays samples from ThumbJam and Korg Gadget - and also I am using Group the Loop (or Loopy) to loop the actual guitar tones as well...  Need lots of foot switches to control all of that ;)

Use -10dBv for any amp.
 
it is a lower level and closer to the instrument level that you need to send from Helix's SEND socket to the HT5 amp front input.
 
+4dB might cause clipping, because you need to set the Helix's Send/Return to instrument level (in Global settings) or otherwise if you choose LINE you'll be putting way too much signal into the front to the HT5 - which is expecting to see a guitar signal powered by the swipe of a plectrum.
 
If you choose +4dbU on the HT5 then the signal coming back into the Helix's RETURN socket from the HT5 amp's SEND will be a LOT louder than what you are sending into the front of the amp.   I already find I need to turn down the RETURN level on the Send/Return block to about -15dB when I use -10dBv to maintain a similar level to when the Send/Return block is bypassed.  If you used +4dbU you'd probably need to turn it down 25 or 30dB.

I think every amp is different when it comes to using 4cm.
 
The trick to setting up 4cm properly is-
1) make sure the Send/Return loop is set at INSTRUMENT level not LINE to begin with.
2) Adjust the SEND level (Knob1 on the Send/Return block) so that the input level from Helix is the same as when you plug your guar directly into the front of the amp.  You can just unplug the cable from Helix Guitar in and plug it into the front of the amp to check (removing the one that's there already that comes from Helix's Send socket) .... LISTEN for a bit to get the level in your head -- then plug your guitar back to Helix Guiar IN and replace the cable from Helix's Send, and then Adjust the Knob 1 SEND so it's the same level as the dry guitar.  (REPEAT as necessary)
3) Adjust the RETURN level control (Knob2 on Send/Return block) to match the level you get with bypassed Send/Return block level and the Helix amp activated instead - assign both the Send/Return block and the Helix amp to the same footswitch and you can A/B between them.
 
Hope that helps!
Rick