Connections?

I have a relatively new CD player (Marantz 6006), a 7025 Marantz power amp, and an older vintage 2230 Marantz receiver (I use as a pre-amp). I want to compare sound using an external Schitt DAC. I've been using RCA cables from CD to Pre-amp to Power Amp. I want to use a digital coax from CD player to the Schitt Modi Plus DAC. Where do I go with the RCA's from the DAC? To the Main IN? Would I then keep the Pre-Out to Amp RCA connection? Please assist with recs for me. Thanks!