Wait up. The Codex can be used in preamp output mode with USB input; that’s one of its features. Check the manual https://www.ayre.com/wp-content/uploads/2018/05/Ayre_Codex_Manual.pdf
You’re making the assumption that the brittle sound is due to the MacMini source, which might be the case. However, it might also be that the Codex is to blame. I’d try to determine which (if you can get something alternate on trial or loan).
In the meantime, try running the Codex straight into your power amp using its preamp mode; you will find out if it’s a Codex issue with your preamp, at least. Most Ayre gear (well not just Ayre) also sounds heaps better using balanced outputs.
You’re making the assumption that the brittle sound is due to the MacMini source, which might be the case. However, it might also be that the Codex is to blame. I’d try to determine which (if you can get something alternate on trial or loan).
In the meantime, try running the Codex straight into your power amp using its preamp mode; you will find out if it’s a Codex issue with your preamp, at least. Most Ayre gear (well not just Ayre) also sounds heaps better using balanced outputs.