I have a Sonos Arc with a Sub and two Play:1s as rear surround speakers, and I love the setup. If you set Music Playback under the Surround Audio setting to Full, when you play stereo music through the Arc, you will get full stereo audio from the front channels of the Arc AND full stereo from the two rear Play:1s. And when you watch movies or TV in 5.1 audio, the two Play:1s will act as rear surround speakers which will give you a more immersive audio experience.

The One SLs produce exactly the same audio quality as the Play:1s. So you could get another Play:1 to use with the current one you own. You can probably find a good deal on a used Play:1 on eBay.

If you are on a limited budget, most users would recommend getting the Sub first, then adding the two surround speakers later. But if you have the money, I would go ahead and get the Sub and Play:1 for a complete 5.1.2 setup. You won’t regret it.

The Sonos Amp is only necessary in this setup if you want to power a pair of passive speakers and use them as surround speakers with the Arc.

I have One SLs and Play 1s as surrounds on different systems, can’t hear any difference in the sound. I’d lean to a good Play 1 off the used market to add to your current Play 1 for surrounds.

The Sub is expensive but it really makes a difference, think about it if the money is available.

If you need to replace your Play 1 that you moved to a surround either get a Sonos One or a One SL (without the voice options) depending on your needs.