Multi-Output device issues when using an interface

My issue is a crackling noise with my audio when using “multi-output device” in Mac OS X (10.9.5)

I'm trying to capture my desktop audio for streaming / recording for Twitch. I've downloaded soundflower as I'm aware Mac doesn't have any default drivers to do so. I'm also using a TASCAM US-1200 Audio Interface with my headphones and microphone plugged into that.

I've created "multi-output device" in Audio MIDI Setup, added both soundflower and the TASCAM audio driver, so this setup outputs sound from both devices at once. Problem is that there is a 'crackle'.

Whatever output is set as "Master device", it does not suffer from this 'crackle', but the other does. Playing with sampling rates and drift correction didn't seem to fix this. Other weird thing is, if I replace my audio interface in the "Multi-output device" with my Mac's "Built-In Output", then there is no issue.

Whilst this is a workaround, using the "Built-In Output" stops me from easily adjusting my headphone volume and causes more problems if I eventually want to look into capturing Pro Tools sessions for streaming in OBS. Have also tried using IShowU but this does not resolve the issue.

Any help would be greatly appreciate - or even suggested forums to throw this in?