Is this a boolean parameter?
No, it’s a numeric one. Leave It open or a list of integers (GPIO port numbers).
Success!
The distortion was from trying to use pin 35 (wiringPi value 24) for the INT_A connection. The hifiberry digi+ must be using this. Instead I changed the INT_A value to use pin 38 (wiringPi value 28) and redirected the wire from pin 38 on the cable to pin 36 on the all in one module. Now I have no distorted sound and working rotary encoders and switches 
I am still experiencing noise when rotating the encoders, and strangely when using the sustain button via my midi keyboard ONLY when the sustain dial is visible on the screen. See this video to see what I mean:
Very close to having a perfect Zynthian with digital audio out! Hope this post helps someone else in the future and thanks everyone for your help.
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It’s the screen that’s causing this interference with the Hifiberry digi+!
I’ll see if there’s any way around this otherwise can anyone recommend a small HDMI touchscreen? 
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I think I’ve killed my all in one module, by accidentally misaligning the connector. It was working earlier but now none of the dials work.
Is there any way I can debug / test it? I have a multimeter.
The easiest way is to replace the MCP23017… hopefully you havn’t killed the pin on the Raspi… Can you try to run i2cdetect again? If the MCP2301 is killed you should not see any device in 0x20 reported.
If you see a device, the Raspi pin maybe killed. You can try to use another free pin (perhaps 37?) and change zyncoder data for INT_A.
Regards, Holger
Hi…as per my experience the MCP23017 on the AllInOne board seems to have 0x20 as address. I don’t know what address the Digi+ has. But you can try by starting without AllInOne and connect via ssh to your Raspberry.