Routing MS USB out to an aggregate Mac out

Hi, I have a Model Samples and a UAD Apollo, I would like the MS UBS out to route to the UAD Virtual 1&2 (this would free up two analogue console inputs that I currently have to use). I have tried to do this by creating an aggregate device on my Mac and adding the Model Samples to it, but then it only shows the option to output to the two MS outs. I have tried adding the UAD to it, this then shows all the outs of the UAD, I select Virtual 1&2 but the MS does not play through them, it still only plays via it’s own analogue outs. Has anyone successfully managed to get the MS USB audio out routed to another device (not DAW inputs) on a Mac? Thanks