I really enjoy the Onkyo C-7030 as well. It's in my secondary system and it's a rock-solid performer that (IMHO) easily keeps pace with or beats many higher-end players from the '90s and '00s, for a fraction of what those cost new, and for less than what many of those still go for used.
I would not, however, depend on any reasonably priced CD player as a direct headphone source for critical listening. It might be fine, but it might not. Power and output impedance are a big deal when it comes to headphone amps/jacks. If you have high-impedance headphones, a lot of built-in headphone amps/jacks can sound anemic. You might have luck listening to the CD player via your recapped 2230's headphone jack - you'll still be getting the benefit fo the CD player's DAC, and you'll also be getting the Marantz's well-regarded sound in the chain too.