The only improvement is doubling of the power if that is a usable benefit.

I feel that AMP sounds better, as do users I’ve spoken to. AMP is more robust when dealing with “difficult” speakers. Some high end speakers have an impedance dip that will fluster a ZP100 along with run of the mill receivers and amplifiers. AMP will deal with most of these very nicely.

It may not seem fair, but some of the improvements attributed to installing a new amplifier are due to the accidental cleaning of the speaker wire connections.

By usable benefit, I refer to people blessed with difficult speaker loads; the art/science of designing speakers to be amp friendly in the sense of these not exhibiting impedance drops/high current draw at certain frequencies seems to have become rarer. For the OP if there is nothing wrong with the existing set up, all he may need to do to make sure that nothing has been lost over the years is this cleaning of connections!

While perhaps not qualifying as “musical quality” per se, but I think the additional power  for the current Amps makes connecting two pair of speakers a much better option than the Connect:amp or ZPs.  I also think that the HDMI-ARC connection adds a lot of versatility.

I have no idea what your existing Hi-FI system is (stereo amp, home theatre setup, etc), but it may be that a Sonos amp can replace both your ZP100 and Hi Fi system, perhaps with an impedance matching switch in between your two zones.

Thanks.

Existing is a reasonable AV receiver by Primare (SPA21) with Anthony Gallo ADiva sphere speakers and a REL Storm sub woofer - 5.1 system. 

Not planning to change this… so may get a Sonos Port to provide input to hi-fi. Current Sonos ZP100 is powering another pair of ADiva speakers in another room. All the satellites are nicely wired with cables embedded in wall. Sonos would require power to wall mounts, which tends to put me off. Very happy with sound quality from main AV setup, including for stereo music.

That kit sounds better than reasonable! If all you want for the hi-fi is being able to play music streamed from the net, an Echo Dot wired to the inputs of the hi fi will surprise you by sounding as good as any other source you may have tried for such music. What the Port will do for a much higher price is provide integration with other Sonos kit, but if that isn't a requirement, a Dot is all it takes. And is so cheap that it can’t hurt to try; even a used older generation Dot is good enough. It can work just as well via the phone to select/start music play, if voice isn’t your thing. 

Thanks. Will try the Echo Dot, just for kicks 

I like the idea of Sonos integration and do sometimes join two or three zones - mainly when I am on my own in the house doing chores - not for serious listening.

That is how I established proof of concept, with that and a Spotify membership. I then changed the Dot for an Echo Show 5 that also added album art display of the music being streamed, for a not a lot more money. Unfortunately the latest versions of the Show do away with the needed audio out jack, so this can now be done only using earlier models of the Echo Show 5/8.

I then moved other rooms to a similar set up using Sonos amps as dumb kit, using the Echo grouping feature for integrating zones when needed, with album art available in addition to voice control when desired, for all zones.

The signal quality from the Echo audio outs is as good, to the ear, as those from any other input device many times the price. Modern kit can do all that legacy kit can, at a fraction of the price, where sound quality is concerned, if one can move beyond the psychological baggage attached to higher price points.

On the other hand there are those that will only use a Sonos Port if they can insert a “audiophile” DAC between it and their HiFi.