I’d lean to keeping what you have and adding a Port until you are ready to scrap the old gear.

EDIT: To clarify the above is all you need, the below are expansion options.

Then I’d lean to an Arc for the TV and adding a pre-amp to your turntable and hooking it to your Port.

Adding a Sub and a couple 300s as surrounds would nicely complement the Arc at some later date.

Thanks for the reply. The setup you described is outside my budget for this project. 

Hi 

 I think you may have been a bit over -whelmed by @Stanley_4 ‘s answer/suggestion.

If you want to play the audio of any source that is connected to your Denon you will need a Sonos Port. The Port would connect to an available RCA Line-out on the Denon to the RCA Line-in of the Port. You would then direct the audio to the Sonos One’s as the default speakers.

To reverse the process and play Sonos streamed content to speakers wired to your Denon you would connect the same Port to a RCA Line-in on your Denon connected to the Ports Line-out. 

The Port can handle both connections simultaneously.

Adding a a Beam2 or any other Sonos soundbar to your TV is a separate option that you can purchase when your budget allows. 

The above said your initial outlay is $449 (Port) or $499 (Beam2). If you were to choose the Beam2 option...surrounds and/or sub can be added later on when your budget permits. 

Thanks, @AjTrek1 !  I have a few follow up questions:

1. Do you know if the setup you described would work with a Connect Gen2? 
2. Would the audio quality of a Beam2 or Arc be significantly better than my decent stereo speakers (PSB Image B15’s)?
3. Is it possible and/or advantageous to complement the soundbar with the dumb stereo speakers? This post seems to imply that this would be possible with an Amp but not a Port. 

Thanks again. 

Hi

The Connect will work the same as the Port. Click this link.

The Arc and Beam2 are designed to combine Left/Center/Right channels in a home theater the same as if the channels were separate wired speakers.  Your current setup with left/right speakers obviously covers the left/right channels. The center channel you hear; which is primarily dialogue when watching a movie is called a “phantom center” as it's a combination of left and right.

To say that a Sonos sound bar would sound better is really something you would have to try for yourself. In most scenarios people purchase a Sonos sound bar for one of two reasons:

• Upgrade from the TV internal speakers
• Eliminate wired speakers for a cleaner look

Using a Sonos soundbar in unison with a stereo is possible if you have (on the TV) HDMI and/or Optical for the Sonos soundbar and a RCA out to Denon RCA-in. However, you would not want to do that as it would create an echo effect as well as make volume adjustment near impossible. So the short answer is “no”.  

The post you asked about the OP wanted to use a Port to drive in-ceiling speakers which cannot be done as the Port has no amplifier. An Amp which can in a home theater would be subject to the soundbar for surround information.

The Amp could drive in-ceiling speakers as an independent set  (room). It would act the same as any Sonos speaker not associated with a Sonos soundbar.