The Verge was shown an image of the unreleased subwoofer and has confirmed branding.
The main thing that distinguishes the Sub Mini hardware is an elongated, pill-shaped vertical cutout right in the middle of the device. It just so happens that the regular Sub has a rectangular cutout, so Sonos seems to have settled on this both for functionality and as a design trait of sorts. In the case of the Sub, there are acoustic ports on both sides, so it would follow that the Sub Mini will have a similar internal layout — albeit with smaller, less powerful components.
While the site wasn't able to obtain release timing or pricing details for the Sub Mini, it will be priced lower than the Sonos Sub ($749) to match with either the Sonos Beam or Sonos Ray. It's possible that the Sub Mini could launch before the end of Sonos' fiscal year which runs to September.
Take a look at the render below...