Master clock in Audition?

I hate to say this but if you can find a way to do some blind testing you may well find that it IS redundant.  Of all the things that effect the perceived quality of your system, the output DAC is near the bottom of the list, well behind microphones, mic pre amps, input DAC, effects you use in editing, monitoring headphones and speakers, and the acoustics of your room

Very briefly, the big advantages of ASIO are a mix of lower latency input monitoring and a complete bypass of Windows audio which is a bodge made more for games and watching Youtube.  In most cases, it's a far more reliable and easier to use system if you can use ASIO.

You might find that it would be better to use ASIO and route all your other sounds (music, video, games, whatever) through that.  In effect that's what I do although I try to keep my audio computer as clean as possible.

As for master clock, ryclark's suggestion makes sense but, as I say, you likely won't find any difference and Audition (at least on my computer) defaults to the output and since it works fine that way, it's one less thing to fiddle with when you have a lot to do.  Like me on Thursday...but I shouldn't complain about being busy.

I would have said the other way around for recording. Use the source device as the Master clock and let the output device follow. If your Focusrite is always connected then I would leave that as Master only using the DAC if playing back without the Focusrite connected.

But may I ask a question why aren't you using the Focusrite for both input and output to save any confusion? Aren't there problems with both devices connected using the ASIO drivers if you are recording via the Focusrite and monitoring from the DAC?