I think the Master section of the Delta 200 is identical to the DLX I have, in which case you can achieve what you want by using... the 2-track returns.
You have the analog outputs of the Finalizer to work with since you are only using the AES output to feed the LIO.
In other words:
Control Room/Monitor Level switch Up >> Main Mix
Control Room/Monitor Level switch Down >> 2-Track Finalized Mix
You might need to do some gain-staging on the LIO or perhaps within the Finalzer itself in order to precisely gain-match the unmastered/mastered signal (if that is your aim) as I don't think there's an easy way to adjust the input sensitivity of the 2-Track Return without a desk mod.
Master Inserts will of course work well too and are designed for this sort of application - just bypass the Finalizer to hear raw mix. Will also unbalance the signal though and reduce your SNR. For sure if you plan to do analog master I/O fades on the desk you will absolutely need to use this method.
You could also hand over all monitoring duties to the MIO Console which has the most incredibly sophisticated software monitoring routing options that can handle almost anything.
My mixing, mastering & monitoring chain are all analogue and therefore a bit more convoluted - suffice to say that these EMO Line Splitters are real life-savers. And I do indeed dedicate 2 of my 24 input channels to the cause, as JLast suggests - these two channels have their input trims way down compared to the other 22 channels to cope with mastered material.
Last edited by James Lehmann; 16th March 2021 at 08:48 AM..