Best system for classical music … and possibly TV.

Sonos say a stereo pair of Fives is their best stereo music setup. Add a Sub to really set them off. 

Otherwise, your Amp has an hdmi connection for tv input. 

If you can find a way to access the used kit market, you can get very high quality passive speakers for the Sonos Amp, for less than USD 2000, that would sound as good as, and probably better than any Sonos set up, for music.

If you cannot, Sonos is the way to go assuming that you live in a place where you can exercise the speaker return option if not satisfied.

Listening to music you like in the place you intend to listen to music is the only reliable way to assess speaker sound quality. This may not be possible with used kit, although passive speakers are the most reliable parts of audio kit where service life is concerned. With used speakers therefore one should be familiar with the sound delivery of those being bought, via prior listening.

A pair of Fives is going to sound pretty good and be the cleanest solution. You might want a Sub if the low end is less than you want though.

The Amp and good non-Sonos speakers are a possibility as are a non-Sonos amp and speakers fed by a Port. The Port gives you the option of shopping for more power and possibly some active equalization in the amp you pick, but less simplicity of use.

It’s unlikely that any soundbar will yield the type of stereo image that you like. Certainly, none of the SONOS soundbars will do this.

FIVE’s are not appropriate for front surround use.

With regard to FIVE’s replacing your current stereo speakers, only you will know what sounds “best” (to you).

A SONOS AMP and your current speakers could be used for front surrounds. AMP will create a phantom center channel. This may or may not work well for you, depending on your speaker separation and imaging quality. For speakers that image well and are close to the TV, an actual center channel is redundant.

I don't think that the Amp has the power to deliver in the low end that you'd want for classical. If you went that route, I'd advise pairing it with a Sub.  If you want seemless TV, your only option is a sound bar, like the arc, but that's not the best for hifi stereo.   

In my Livingroom, I have a Playbar for TV, and a Connect with a regular Hifi stereo system for music. 

The CONNECT:AMP had 55 watts, the newer replacement Sonos Amp has 125 watts. While it depends on which speakers you’re driving, I’d suspect that in most applications, 125 watts is more than enough. 

In both devices, there is a line level subwoofer output, the device itself only feeds that data, it doesn’t amplify it.

What is the watts per channel, rms calculated, delivered by the HiFi? More than what the Sonos Amp delivers, which is 125 watts into 8 ohms and 250 into 4? What makes you think that Sonos Amp is not HiFI stereo in the sound quality it delivers?

Watts is only part of the picture.   A power amp must also supply a good bit of current to drive the mass of a woofer.   That requires larger transformers and capacitors than you could fit into the Amp.  My power amp is rated only for 100 watts per channel but it weighs at least 20 pounds and is several times the size of the Amp.  The sound difference is night and day.   There's no getting around physics.

For casual listening, I think the Amp is fine, but it's not for audiophiles unless maybe if you pair it with a Sub. 

Please don’t tell the Carver M-1.5t that I used to own that, it will be sad about it’s puny 15 pounds meaning it can never make it’s rated 350 Watts RMS continuous and 750 RMS short term peak power.

The new Sonos Amp is so many generations ahead of the Carver.