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.

I recommended the Sonos amp to a friend of mine and he’s very happy. He uses it with a pair of high end Canton speakers (forgot the exact model) mainly to listen to music and as an on top feature for watching tv. 
The center simulation works well for him but imho depends on the connected speakers. 
I definitely would give the Sonos amp a try.

Thank you everyone for your responses. I should have said that "amp” that I have is the old version, circa idk 2004, no HDMI, just ethernet RCA and subwoofer. 

I’d trade that old ZP in on a new Amp, you should get the 30% discount and still be able to keep and use the old ZP.

Not A bad idea, TV can connect directly to it, I gather, through HDMI, but of course I'll need to spring for a pair of good speakers … tempting!

I would first get speakers that work well with your existing amp for music. And with 55 watts per channel into 8 ohms and 110 into 4, that amp delivers as much current as any other alleged HiFi amp with the same numbers in its spec - if it did not, that spec is false and I do not believe that to be the case. 

So you should find many speakers that can sound good with that much power to drive them, as long as your speakers are not too far away from the listening areas. And any speaker pair that is getting different signals for left/right channels will deliver a stereo image, and a phantom centre channel effect for TV - if the TV is placed centrally between the two speakers, the dialog will appear to come from the TV, if you sit in front of the TV, about the same distance each of the two speakers.

The next thing to do is to wire the TV sound to the amp via RCA wire - the TV will have to have either audio out jacks or a headphone jack to do that. The only possible problem you may face is lip sync - the  sound may come later than the picture. But this often is not intrusive or even noticeable - I have not found that to be the case based on the TVs I have used. If you also are in my boat, you are all set, there is nothing more to be done.

I would look to replace the existing amp only when the old one dies. Or, if the lip sync issue is intrusive in your case. Do not assume that it will be so, without seeing for your self.

PS: when I was exiting my audiophile decade, I replaced a Quad pre and power amp of 140 watts per channel by the same amp you have, driving a pair of large Harbeth speakers. I found no difference in the sound quality of the music after the change. Most of audiophilia is psychological and does not stand up to objective testing.

@Kumar thanks I'll try that first, would rather invest $ in new speakers than new amp and/or Beam or Arc. I'll have to take the TV off wall to see all of the I/O options, just terrible ergonomics-- the new Frame is great for this, one slim waite carries all signals, and power, to a rather biggish black box, but still, don't have to remove TV from wall.

Good luck with that because TVs are a mess of non standard connectivity at the rear; and wall mount can add to the challenges. Plus there is an issue for TVs that do have the necessary I/O, the TV sound is not automatically muted in all cases where the TV speakers need to be taken out of the loop, as they need to be for best results when other speakers are in use.

If you can get the system to work per above, with the existing amp and you are getting good results for music and TV, you have another upgrade path - add a powered subwoofer to the amp connected by a wire to it. The wireless Sonos Sub probably is no longer compatible with your amp and also is not cheap. This will address the need for any lack of power in the amp, or lack of bass capability in the new speakers. Caveat here: adding subs can be tricky, especially to get good results for music and the ideal would still be spending on capable new passive speakers - used if possible to get the best value - with high sensitivity and that have a nominal impedance spec of 8 ohms at least - 88dB or above for the sensitivity spec - that are in large enough boxes but that can still be stand or shelf mounted. I don’t think that the tower style of speaker is necessary, their expensive cabinetry costs/prices means you pay more for the same sound quality obtained for an equivalent sounding standmounted speaker.

It worked! Bought a powered D to A converter, HDMI arc in from TV, RCA out to my connect:amp. No obvious time delay issues, dialogue clear.

Excellent; what speakers are you using?

A very old pair of CELESTION DITTON 100 shelf speakers (over 40 years omg I'm old). Clearly it's time to upgrade, no floor space available so will stick with the bookshelf size. I've read a bit about KEF LS50 Mini Monitor Speakers, but of course can never know until I get them in the room. My room is odd shaped, with TV in a slightly recessed area, facing perpendicular to room length. So the listening area has wall/windows on one side, open on the other. Anyway this should be fun, finding a killer speaker. 

It isn’t necessarily clear; passive speakers can give decades of service and if your pair is working as it should - as judged by how it sounds to you - you may end up having spent money replacing these with not much in return in terms of an equally improved listening experience for the money spent. I suggest taking time looking for deals in the used market where value for money is kept high via audiophiles on the upgrade bandwagon.

Thanks for the advice Kumar–you’re right, why spend lots of money, there’s no rush. Thank you again for the advice!

Spending money is fine for equivalent receipt of perceived value, and if that spend does not increase landfill burden via dumping kit in working condition, for psychological reasons. That was always true, but is more relevant now in these days of what look like stressed out times for the environment.

Enjoy the outcome you have achieved!