Hi,
it is nothing special and a few others have built more integrated units (will dig out a link after this and add it to the post). It is simply a spare centre channel speaker I had lying about unused, but a decent model, a PMC DB1C. I just built another PI with IQAudio DigiAmp+ HAT. The speaker has a split crossover so one channel drives the woofer and the other the tweeter for approx 25W+25W. It is powered from a 19v laptop 60W PSU (also spare and lying around). Roon is set to send a mono signal but you do need to put in some headroom to avoid clipping.It is absolutely 100% reliable and stable. I use Diet Pi with Shareport/Shareplay which just works fine for Airplay use.
With regard to Sonos One comparison, it is rather better. I did used to own a Sonos One but the tubby bass was rather off putting, as it tends to be with small speakers.
EDIT: @CrystalGipsy posted a very useful walk through a while back: