The extra cables to connect it in would have to be expensive and a complete waste of money when you don’t need them with an integrated CD player/DAC. That being said I would like to hear what that Metro Onyx R2R sounds like.
@skyscraper — Cables need not be expensive. All you’d need is a digital cable with the analog interconnects you use with the Marantz, and for purposes of auditioning a DAC you could get a 1m Apogee Wyde Eye cable from Amazon for $40 that is a very good cable (I’ve used it for years and it has beat out more expensive cables). Do yourself a favor and demo an Onyx — again HiFi Heaven has a return policy to do this. At the very least it’ll be very enlightening for a relatively small cost outlay, but again my bet is you won’t return it. Again, best of luck in whatever you decide.