FLX6-GT: Master always in headphones cue when using external soundcard as aggregate device on Mac (Intel)

hey all, I'm getting really strange behavior from my FLX6-GT that just recently started after the update to Rekordbox 6.7.3.

I am using an external sound card: Apogee Duet for iOS (https://apogeedigital.com/products/duet/product-tour)

I used to have the Duet as my main Mac audio device and I would check the "Output audio from computer's built in speakers..." option and everything would work great. Now, when I click the checkbox, it attempts to do it, briefly shows "No Audio Device" and then goes back to just having DDJ-FLX6 and the box unchecked. I've attempted to install the driver it selects when hitting the "Driver" box, but there isn't an FLX6(GT) driver in the folder at all so I just installed the one the link highlights when I click it but nothing has been fixed.

Because of this: I've had to create an aggregate device between my Duet and FLX6-GT. It looks like this:

I then setup the output channels in Rekordbox 6.7.3 like this:

the Master output is perfect and there is no problem there. 

The problem is what I'm hearing in the headphones when I cue: I am able to hear the master output 100% of the time. I am also able to hear whatever I cue, but I ALSO hear everything coming out of Master and I cannot figure out how or why or what I can do to stop it.

I have tried an alternate approach of solely using the Duet and routing Master to 1+2 and Headphone to 3+4. This also works, but the problem persists of hearing the Master in Headphone Cue 100% of the time.

Any help or suggestions are appreciated! thanks.