Need help designing a setup for my garage

I don’t know how familiar you are with Sonos home theater. So, in the interest of providing a thorough recommendation I’ll proceed under the assumption that you’re a novice (no offense intended) 😊

Sonos home theater typically involves a Sonos Soundbar such as an Arc Ultra, Beam 2 or Ray *. Those products are the lynch-pin for peripherals such as surrounds (Era 300’s, Era 100’s or Five’s) and subs (Sub 4 or Sub Mini). **

Those components when Bonded to a soundbar cease to operate as independent speakers. Meaning they can no longer be used separate of the soundbar. All audio information is sent to them via a hidden 5Ghz channel emanating from the soundbar. 

The music setting for surrounds is Ambient or Full Stereo. However, when playing music, the soundbar will play as well. Some like the effect of being in the middle (front to rear).

The other option for home theater without using a Sonos soundbar is a Sonos Amp to drive two speakers flanking the TV; thus creating a “phantom” center channel. IMO, if pure stereo is your preference the setup is better than a Sonos Soundbar for music. I have an Arc Ultra home theater setup; but use two Five’s upfront for stereo music.

There are two options for surround with a Sonos Amp:

1. Use Era 300’s, Era 100’s or Five’s. They will communicate with the Sonos Amp over the same 5Ghz channel mentioned above.
2. Use another Sonos Amp to drive two 3rd party speakers that communicate with the main Sonos Amp via 5Ghz channel.
3. However, as with the soundbar those speakers (in either configuration) cease to be independent and produce audio via the pleasure of the main Sonos Amp.

So, much for the tutorial on Sonos. Let’s talk about your preferred use case:

• Sonos Amp driving two 3rd party speakers upfront and a wired 3rd party sub.

You mentioned possibly using Era 100’s or Five’s as rears for surround. That’s fine. However, there are considerations

• You already know that the rears will be governed by the Amp which means the sound for stereo music may be unbalanced versus the Yamaha’s.
• Surround sound for movies should be OK

My recommendations using the Sonos Amp to drive speakers up front with surrounds in rear are:

• Use existing Yamaha four speakers
• Two front
• Two rear driven by a 2nd Sonos Amp

Another option as the Yamaha’s are 15 years old:

• Purchase four 3rd party bookshelf speakers (or larger depending upon budget)
• Purchase 2nd Sonos Amp
• Use two front driven by 1st Sonos Amp with Wired sub
• Use two rear driven by the 2nd Sonos Amp.

Here’s another option with positives and negatives

• Use Yamaha’s or newer 3rd party speakers up front
• Use Yamaha’s or newer 3rd party speakers in rear as stereo pair driven by 2nd Sonos Amp. 
• Era 100’s or Fives’s as stereo pair (2nd Amp is not required)

Positive:

• Rear speakers can be used independently or Grouped with the front speakers for music

Negative:

• Rear speakers Grouped with front speakers for TV audio creates an echo effect caused by a 75ms delay.
• Rear speakers cannot be easily used as surrounds as a reconfiguration is required and the reverse to get them back as a stereo pair. Very tedious and time consuming.

Other:

• All connections to TV will be HDMI that includes Sonos and boxes like AppleTV, Game consoles etc. 
• Airplay is only to send audio from an Airplay capable device (Phone or Tablet) to a Sonos speaker. No audio frequency can be directly sent to a sub.

Well, that’s all I have. I hope it helps you make an informed cost-effective decision. 

* Ray not recommended as it’s too small and only has optical connection

 ** A 3rd party sub wired to a Sonos Amp or a Sonos sub Bonded to Sonos Amp, soundbar, single speaker or stereo pair cannot be used to supplement audio originating from other Sonos  sources.