Hi nolagt,
I am also a big Sonos user. I have a Sonos Connect in my HT feeding my HT amp through optical output. In my casual listening environment in the living room I have the Sonos Amp driving a small pair of Polk bookshelf speakers with a Polk sub - from my 1st entry level HT setup. Plus a Play 5 with plans for more zones in the future.
I think you have your worst case scenario already covered - using the Sonos to drive the pair of Algonquins outdoors. If you are happy with the volume you can achieve there then I think you'd be OK with the M3's indoors. The Algonquins are essentially outdoor M3's. Of course there are differences between the design as you noted, however I think your biggest difference for pure volume is outdoors vs indoors, so if your happy outside I you should be good inside.
Of course another option is get the Sonos Connect for much cheaper than the amp model and buy a cheap but more powerful 2 channel amp to drive the speakers.
Hope that helps.
