Multiroom setup using existing speakers and amplifiers.

Hi.  The only route to doing this with Sonos would be to use Ports, which would be very expensive.  I could not possibly be a bigger Sonos fan than I am, but given where you are starting from, and what you want to achieve, I would buy four Amazon Echo Dots, preferably when on one of their frequent offers, and connect into an input on each of your amps.  

This answer assumes that you live in a country where Amazon Alexa is available and that you are happy to use Alexa.

Further point - I believe Echo speakers are Spotify Connect enabled, which would mean you could play to the speakers using the Spotify app on your phone.  You had better confirm that though.

If you get a good offer on the Echo Dots this might cost you only about £/$120.  Worst case £/$200.

I am a big fan of the Echo devices for all they can now do for a legacy style HiFi system that makes Sonos redundant, while retaining legacy kit sound quality - EXCEPT for the kind of use case you are referring to where no one still serves that case better than Sonos does.

The Echos will serve, but it will be nowhere as slick/fast as Sonos. Unfortunately the entry ticket to that is the expensive Port for each room. Perhaps a couple of Dots can let you see if they suffice, but they probably will not if the use of multi room is extensive.

If all systems use a 3,5mm jack a Sonos Port connected to it (via an RCA to minijack) in every room would get you the same (or different) music in every room. You’d have no volume control though.

Yes you would, with the Ports on variable level setting. And the receiving kit can also have a Aux in socket pair for this to work.

Ah, you’re right. Didn’t realise this as my Connect is on fixed.

Thank Y'all for the quick and thorough responses. Yea, $450 per room for a port, may be more than I was looking to spend. Two of the rooms have the nicer stereo equipment, where as the other two rooms I could consider switching to Sonos speakers. If I got two ports and then two Sonos speakers I could then stream in all 4 rooms, correct? Also would this setup would allow me to have variable volume in each room? That is definitely important to me. 

As far as sound quality goes how do the echo dots compare to the Sonos port? Are both options just going to be the "very high quality" Spotify streaming quality sent over comparable 3.5mm outputs? Or does the Sonos port offer a significant upgrade in the quality of its output signal over an echo det? Also does the 4 echo dot setup offer variable volume control in each room? 

Finally, I have two existing Google home minis. I have been frustrated before trying to get them to play the same music in two separate rooms. Maybe I was using it incorrectly (I got quickly frustrated and didn't really try to figure it out as it wasn't important at the time.) but is it possible to achieve the same thing as 4 Amazon echos with 4 Google home minis? 

Thank Y'all again, this has been very helpful, Will

To the first, yes, this is a good approach and yes to all the rest of the questions.

I have used both the earlier version of Port and Dots as sources and I have found no audible differences - what the Ports offer is much slicker multi rooming, that is for sure.

No idea about Google Minis. I have Echo Show units wired to the line in jacks of my Sonos kit, and I find myself using the Sonos controllers for multi rooming. All other times, I use the Echo Show - either via voice or via Spotify Connect, bypassing Sonos controllers.

The sound quality will, I believe, depend much more on the audio equipment you have than on whether you use a Port or a Dot to feed it.  The Port would be better, I am sure, but whether enough to to justify the extra cost is a very individual and personal decision.

@Kumar is correct that the Sonos user experience would be superior.

I use the multi room feature extensively, just not for multi rooms where I don’t use it at all!

To clarify:

My 6 zone set up has 5 zones for a open concept living/dining/kitchen/bar and adjacent patio. The sixth zone is the bedroom.

I never feel the need to have all 6 zones playing the same music - in fact, my bedroom is a Echo wired to a Sonos Connect Amp that always plays standalone, and the Sonos unit could well be any other third party make amp.

However in the open concept space I often need more parts of it filled with the same music, in perfect sync, but with separate volume controls to fill as much as the space as needs filling so that the music does not sound too loud in any part of the space. So I may group as many as 2 to all 5 zones there from time to time based on how the space is being used and Sonos supports this better than the wired to Sonos Echos presently can.

If I were in a house with rooms that are in fact rooms and not zones in a large open space, I doubt that I would feel any great need for multi room, and, as a consequence, for Sonos.

I therefore suggest that the OP be sure that his need for multi room is real and will persist once the novelty wears off.

As to the sound quality question - if analog out of the Port is used, I see no technical reason for sound to be better than from an Echo Dot. And neither do I hear any. If the digital out on the Port is used feeding a very high end DAC, sound quality may be better, but I have not seen any such surviving a strict blind listening  test.