I have a pre-production unit, but the terminals on the back of mine are  - + - +

I've done a little more testing with music and volumes.  We normally listen at low volumes (under 50%), but I do agree that the volume doesn't start to ramp up dramatically until you get to 75% or more.

Sonos knows exactly at what volume all their devices are played at so they have statistics on the average volume used.  Maybe they've implemented this on the new AMP so the volume isn't linear and gives more precise control in the lower 0-75% which gives the impression the AMP isn't as loud as the old Connect:AMP?

When I did A/B/C comparison with the same speakers connected to the AMP, Connect:AMP and Marantz AVR I was focused on sound quality at my normal listening volume so didn't do any comparisons of max volume.