Are there ANY user-adjustable audio settings available in HomePod?
Can I adjust ANY audio quality settings on my new HomePod (other than volume up/down)? e.g. "More bass," "less high-end," "more 2K" etc. etc.
No it’s automatically adjusted to original artist’s intent. EQ that you use on iPhone/iPad music settings aren’t adjusted on HomePod because with airplay it accesses the direct audio file on the phone or cloud music or movie audio that has best audio quality. If you are having problems still I would recommend updating to iOS 12 or just unplugging for like 10 seconds and reply it in to see if the restart takes care of any issues?!