You should be able to setup, your monitor mixes, etc. from within cubase, however depending on your circumstances (i.e. heavy track count/CPU load), it may be more beneficial to monitor outside the box, via your trusty old 24 track mixer (Even with the best computer setup, you'll get lower latency with the 24 track).
I have kept my old analog gear even though I have moved over to the digital realm. I'd say keep the mixer, and invest in the control surface too.
Remember, that 24 track mixer still has a use in life (Studio monitoring, Live gigs, location recording, as a backup for digital gear which always seem to break at the worst time, etc.).
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