Sonos + receiver + L/R speakers + turntable

Let's see, I'll take one thing at a time;

SInce you want to continue to use your current speakers and amp, you're going to need a CONNECT, which would basically be connected in the same manner as a tape deck, with RCA cables between the two devices. Anything you stream through the Sonos app would then be in sync with the new Sonos speakers yet to be purchased.

However, the issue will be when playing the turntable, which will be connected to the amp, as it is now. The issue will be that there is only one set of inputs on the CONNECT, so when you play a record, the two different kinds of speakers (the ones connected to the amp and the Sonos yet to be purchased) won't be in sync. However, if the speakers are in different rooms, and you can't hear both of them at the same time, it may not be an issue.

Now, if you don't want to use the wifi bandwidth in your house, you just need to run an ethernet cable between your router and the CONNECT, which will put all of the Sonos speakers on SonosNet, which is its own separate parallel network. In fact, if it isn't convenient to wire the CONNECT, you could just run an ethernet cable to any of the Sonos speakers, and it will create the SonosNet for you.

A Sonos connect does make the most sense. Also add a Boost if your current Sonos system doesn't already have one. That way the whole Sonos system will run on its own isolated Wi-Fi network rather than piggybacking on the house Wi-Fi.

How you hook up the Connect depends a lot on the model of Denon AV Receiver and whether you want anything other than the turntable and server piping through to the other Sonos speakers in the house.

Very few AV receivers from the last 3-4 years have a set of analogue record out connectors. This is what used to be common when hooking up a cassette deck / minidisc deck / CD recorder. There'd be a set of connections which had both the regular analogue inputs but also a set of analogue outputs; so, 4 phono / RCA / cinch sockets (2x in, 2x out - stereo). If your Denon receiver is like most now with just analogue inputs then here's what you do:

Plug in the turntable directly in to the Line IN sockets on the Connect. This gives you TT & media server in to the Sonos system and the rest of the Sonos speakers in your house. Next, hook up from the Line OUT on the Connect in to any normal Line IN on the Denon; so that would be CBL/SAT or CD or Game etc. Now you've got Sonos & TT in to your Denon. Remember to do the input assign on the Denon so it knows where to look for the Sonos signal.

To play the TT through the Denon switch the receiver to the Sonos input, then from the Sonos App select the Line IN on the Connect. Your other Sonos speakers will also have an option to listen to the Connect's Line IN, or your can group the speakers & Connect as suits your needs.