Sonos has released the new firmware version 14.18, which fixes an annoying problem with the Arc and Sub devices.
Normally, a firmware update provides bug fixes, and with regularity, new functionality. That went all wrong at Sonos a while back. Owners of the Arc and Sub complained that after updating to firmware version 14.12 the sound was suddenly much poorer. The volume in general was reduced, coupled with a strange frequency curve. The latter led to a rather drastic loss of bass. The firmware version 14.18 now released by the manufacturer fixes these bugs, as has been confirmed by many users. So update soon, if you are experiencing these kinds of problems on your Sonos speaker(s).