Yes, you could continue to use your current speaker switcher provided it presents a minimum 4-Ohm load to AMP. I expect that this is the case.

You will need a phono preamp for the turntable, and some means of switching inputs. The input switcher would connect to AMP’s Line-In. If your integrated amplifier has an analog output, possibly anticipating a tape recorder, this would be appropriate for Line-In.

You will not be able to individually select or control inputs and speakers using the SONOS controller.

If you have additional SONOS units, such as a PORT or FIVE that have a Line-In, you can connect an analog source to each, then select and play using the SONOS controller.

Would the fact that TS has speakers in more than three rooms (“kitchen, patio, living room, etc.") not change the answer? As I understand it you can only connect a maximum of four speakers to the Amp, so unless the speaker switcher provides amplification, if he has more than four speakers he'd need another Amp?

I’m assuming that the speaker “switcher” has impedance matching capability that the OP would continue to use, else the integrated amplifier would have died.

Just wanted to add that if the existing amplifier has an analog output, they you could leave your existing system as is and connect the analog output to the input of a Port.  The Port can then group with other Sonos speakers you have to play the audio.  Note though that the Sonos speakers would be slightly delayed.  You can also connect the output of the Port to your existing system to have it play Sonos streaming services.  This would not have a delay.

Thank you for the quick responses @buzz @106rallye@melvimbe...Is it safe to assume that both the speaker and input switchers would have to be manually operated? And that the only way to control each input (CD player, etc.) through the SONOS app would be to connect each component to its own Amp? As for the speaker groupings, I'm guessing I'd only be able to manually toggle which rooms would playback the same song, and would not be able to play different songs in different rooms unless I had an Amp for each?

This speaker/input environment got me thinking about WiFi streaming adapters, but I'd really like to be on SONOS considering I already own a few speakers.

Thank you again! I sincerely appreciate your time, expertise, and guidance.

A given AMP plays the same music to all connected speakers. If you want multiple streams you’ll need one AMP per stream. Again, if any of your other SONOS products have a Line-In this Line-In can play in any combination of Rooms.

With respect to remote controlled input and speaker selectors, they are out there. Each will have its own IR controller. There is no possibility to integrate these switches with a SONOS controller. It’s probably a larger “box” that you’d like to keep, but if your current integrated amplifier offers an IR remote you could continue to use this as your input selector.

It’s a DIY project, but it would be possible to develop a phone/pad/computer controller that could deal with the speaker and input switches.