Sonos Amp and KEF LS50 speakers.

I don't really see the point in using a third party amp here unless needed for connectivity features. 

I have found with Connect Amp that Sonos has architected its volume control more linearly than the typical stereo amp that goes loud early on and then does not do much beyond 60-70% other than distort. Sonos needs to be set at 70% to get the same sound levels that most such amps will deliver at 40%, thereby the latter just feeling beefier. But Sonos can be set to 100% and will keep going louder from 70 to 100 without distorting.

I am pretty sure that Sonos Amp hews to the same architectural style.

So a truly beefier amp would need to be 200 wpc plus, to give more than a 20% sound level bump over the 100 wpc Sonos, but the KEF still won't do genuine bass.

Far better would be to add a Sonos or third party active sub instead. That will make up for lack of ability for true deep bass in the LS 50, while reducing power draw from the main amp since the power hungry bass frequencies will be diverted to the sub. In such a set up the Sonos Amp will be more than enough for the inefficient KEF.

And I would first do a comparative listening test of a Sonos 5 pair to see how that stacks up against the LS50 driven by any amp. You may be surprised that better sound quality from the former may trump the LS50 aesthetic, and there will be a lesser need for a Sub.

Fundamentally, ANY passive speaker out there uses decades old tech compared to active speakers like Sonos with their separate amp dedicated to each drive unit, and active crossovers/DSP to make the whole thing integrate effectively, with or without things like True play tuning.

The aesthetic is a different matter, I agree, and is a subjective personal variable where preferences are concerned.

Had a demo of the LS50 Meta with the Sonos Amp as Seven Oaks Sound and Vision and was pleasantly surprised.  I tried a variety of songs and to be honest missed the low end in some songs.

I’m going back next week to re-demo it with a sub attached as well as to compare against the Blue Sound Node2i

Another data point:

Right now, I am listening to some classic jazz and I find that there is little to choose between a Connect Amp+ Quad 11L speaker pair and a play 1 pair + Sub in two zones of my open space living area. If anything, the latter shades the former a little because of the presence of the Sub. 

The Quads are well loved speakers in use from 2002, and still look and perform as well as ever. They definitely go deeper than the LS 50 and the mid range is also up there with my KEF pair, albeit one that is at a much lower price point than the LS 50. 

None of this is news; but the pleasant surprise is that the source for both just now is a Echo Show 8 wired to the line in jacks of the Connect Amp, with album art and voice commanded music from Spotify, with an option to also use the phone app instead of voice. I know that the source is far from Hi Res, and I don't even really care to know what the codec in use is, since the music sounds as good as I have heard from many familiar tracks where the source media were the now obsolete vinyl and SACD players.

So, buy the LS 50 by all means, but not all you pay for may be for better sound quality that will survive a blind test. 

Hi - I’d be really interested to hear how you got on with the second demo, I’m thinking about the same thing at the moment.

As an enthusiast of all things Sonos (and owner of an LS50), I think the LS50 and Sonos Amp would be an unwise pairing audio-fidelity wise. The key with the LS50 is they need more than wattage, they are picky to the amp source. The only scenario I would do this pairing is if convenience is the primary goal. Though given that scenario, you could achieve the same results with a lower model KEF (ex. q150).

What more do they need that the Sonos amp cannot deliver, as visible in the measure spec of the amp? Interesting to know.

I agree that LS 50 is vastly overpriced though, compared to the excellent Q150.

It’s the quality of amps and how the amp is able to handle sudden power demands (ex. when lower frequencies are present) also the sound signature of the DAC that complements the amp. There’s more to an integrated amp than just wattage. Looking at just wattage is like saying the engine of a McLaren 720S is the same as a Hellcat b/c they both output 700+ HP. 

Anyways, I’m sure the OP already made their choice. Also...oh yes the LS50 is sooo overpriced. lol

But the difference in the engines will be apparent in their power and torque curves, correct? These are always part of the engine spec.

So what is it about the Sonos amp that is visible in its spec that is lacking? I haven’t found anything that is.

Correct, but don’t forget you don’t just use a car’s engine, you also need the transmission to put that power down. Same goes for a stereo system, you need to a DAC to encode those 1s and 0s to an analog signal. 

Long story short, no matter how I explain this you need to listen to another system to believe. The LS50 while decent on the Sonos Amp have more to gain with a better amp. Don’t get me wrong, the Sonos Amp is a nice tidy package, but if you are looking for the best fidelity, there are other options.