If you think there may be a fault, that’s pretty hard to diagnose in a forum post. I would recommend that you submit a system diagnostic , and call Sonos Support to discuss it, or post the diagnostic number here for a Community Moderator to pick up.

There may be information included in the diagnostic that will help Sonos pinpoint the issue and help you find a solution.

When you speak directly to the phone folks, there are more options available beyond just the diagnostic analysis.

Managed to get the tuning to work. I think my phone may have been too far from the Arc. Anyway turned everything (including phone) off and on, and tried again. this time did not venture as far away in open plan room, and it seemed to complete. Now need to verify if audio has improved any...

...After tuning, audio quality improved. Bass not super deep, but it no longer sounds broken. I see there are a couple other recommendations in this site to more optimally tune ( e.g. with an ipad vs iphone -  will give that a try).

So glad it was not a fault. Those are some bad default settings !