Not sure what you mean as a ‘hifi’ system. In normal usage, a PLAYBAR, with two PLAY:1s and a Sub would be set up in a home theater configuration, and prove Dolby Digital (5.1) support for data coming in to the PLAYBAR’s digital input, via the optical cable.

I wouldn’t expect it to receive or play Atmos, it doesn’t have the ability/CPU to do so. 

The second PLAY:1 would be a substantial challenge to find, as they haven’t been manufactured for years, having been replaced by the Sonos One initially, and now the Sonos Era 100. You’d likely need to find a used one out there. 

The PLAYBAR is an ‘ancient’ device as well, having been supplanted by the Sonos Arc, and now the Sonos Arc Ultra. 

All that being said, I used two such systems to excellent affect and satisfaction while connected to older TVs via optical cable.

I did, however, have a pair of Sonos PLAY:5s (long since replaced by Sonos Five, and possibly the Sonos Era 300) that I used for music playback, mostly as I preferred wider stereo separation. In my bedroom, I’ve used either a pair of PLAY;3s (again, no longer made) or Sonos Ones.

Adding another Play:1 and Sub is how many first used a Sonos home theatre setup - it was pretty much their first offering. It works. Be careful to get a Sub gen that’s compatible with your app (S1 or S2).

For a better hifi configuration, consider using the Play:1’s in a more conventional hifi setup, in front of the listening position. (Or bite the bullet and get a pair of Eras.) The soundstage will probably be better than you get from the Playbar. Just create a new “room” rather than bonding them as surrounds.