Should I replace my hifi separates with a Sonos system?

So how do you think this more real thing would show up in a frequency response graph? Or in any other instrument that is not subject to pyschoacoustics in the way that nature designed the human brain to be susceptible to?

Voltage/Current = power delivery = higher sound levels. Not higher fidelity, unless by fidelity you mean the ability to go as loud as an orchestra. But then I have heard of no kit that can come even close to equalling the palpable experience of an orchestra, heard live. Or even a small jazz combo, live in an intimately small club. All home audio is a pale copy of that.

You're right (in only my opinion of course) on one point, that is power is louder, and nothing comes close to the real thing, which is why I gave up chasing that ghost. However, trying to make a simple FR graph correlate to what you hear in your room on a Tuesday evening is an impossible and pointless task, and pointless. The only measurements worth making are a frequency response map of your room. SPL meter, test tones, a tape measure and a pen; or a bit of hardware and software to measure it for you. Frequency response graphs are as relevant as £5k / metre cables.

The devialet probably has less distortion, that's what you're paying for. It's evident when comparing for example a B&W 602 S2 with the ATCs. The louder (more realistic) you play, the bigger the gap.
I am not so sure that louder = realistic is all that it is about. Going by what you say, if one was to sit with a good dB meter in a concert hall or a jazz club, and use that reading to set the same sound levels at home in the listening position, the listening experience should be replicated. Particularly where a jazz club is concerned, obtaining the same sound levels will not be a huge challenge for home audio kit with enough power. But there is no kit I have heard that comes even close to the real thing. What then is the gap coming from?

And my question remains: if a frequency response graph is a worthless gauge of speaker sound quality in a room, what is a good one? All I know for sure is that a human and his/her subjective opinion is just that, subjective and therefore far from good enough.

Peter, that's exactly the point. Good gear can make a very good copy of an instrument, providing we can mitigate room interactions (which is why it's more than just measuring frequency response). Try this, record said trumpet player in a given room, then record the speaker playing that recording in the same room.

I have tried it. It's surprisingly close, using my separates system, that was why I bought this ATC speakers in the first place. But, I figured that bring able to faithfully reproduce trumpets or drum kits in my house that were recorded somewhere else is pretty pointless. I concluded that Hi fi is pretty pointless, and largely a waste of money. So long as you enjoy what you hear, it doesn't matter if it's true or not. If I want to listen to real instruments, I'll pay to watch someone do it or play it myself.

I am a convert to the small active speakers + Sub philosophy. High and mid range can be well handled by small boxes that are easier to place in an optimum location, and the Sub does the rest. Provided it integrates well with the speakers in the manner the Sonos unit does.

Umm....with small active speakers that don't do much below 100hz, I certainly need a Sub for music of the highest sound quality in the home, even for the kind of music I mostly play, acoustic jazz. And not having to struggle to do what little they can with these frequencies allows these speakers to then be better at what they do well. The Sub also does a lot to add depth to music played at low volumes much better than the loudness switch does. Another advantage is that placing two full range speakers that both put out a lot of low frequency energy can be a bigger challenge than doing the same thing for one Sub. With the Sub in place, little needs to be done for the satellites beyond making sure that they are aimed in the general direction of the listening area.

But proper integration of the Sub with the satellites is a must if this is to work.

I haven't tried a 5 pair with/without Sub, but there are some here that think it adds value to that as well, for music. But with the 5 capable of decent energy down to 40-50 hz, I agree that the Sub may not be necessary. I don't know enough of the 5 units to say more than that.

Slightly unfair as the NAD is not an amp to drive big ATC speakers
Digressing a little: presuming this to be the 370, why isn't it good enough for these speakers? I ask because I have used smaller NADs extensively in the past and still use the excellent 325 for my TV audio . The 370 was too heavy and powerful for my needs.

I don't remember ascribing "highest sound quality" to it though.....

Sorry, perhaps I misunderstood your "I certainly need a Sub for music of the highest sound quality in the home"

And for the benefit of others that may chance upon this thread and find their way here:
I replaced a USD 7500 hifi system with a play 1 + Sub; a system that was the last of a few I have been lucky enough to own over the last couple of decades. In my living room that can comfortably seat 8 people, I found very little loss of heard sound quality and none to the extent that it affected my listening pleasure - which actually has increased substantially after installing Sonos because I now listen to a lot more music, a lot more often than ever before.

I found the play 1 pair to be close but not equal to the mid range that my legacy set up delivered, while the Sub did a better job with instruments like the double bass in a jazz trio and in providing a better sense of presence overall to the music during low volume level listening. The combined effect was good enough for me to not regret making the change even for a day. Perhaps things would have been different if I had a living room large enough for all the advantages of the legacy set up to come through, but I don't, so that point is moot for me. And even in that case, a 5 pair + Sub would probably be enough to compete on level terms. Not that surprising really, when one considers the many years of difference in technology - passive speakers based kit is technology from the last millennium. And when one analyses the reasons why it is still priced as high as it is, reasons that have nothing to do with objective sound quality delivered.

For completeness, I have to say that I took as much care over speaker/Sub placement as I would over any hifi kit I have used but that was also a lot easier to with the 1 pair + Sub than with any other kit I have used in the past. And finally, Trueplay did the last little tweaking to take care of what still needed to be done to take room response into account.

So to the question posed in the thread title I'd say - it depends - because to start with, the phrase "hifi system" is fuzzy and inadequately defined. That said, I'd say that in most cases there is a very good chance that replacing the hifi system with Sonos can be done without a sound quality compromise. And in every case, i'd suggest that given the potential benefits of a replacement, and given that Sonos kit is returnable in many places if found inadequate at home, the effort involved in listening to an appropriate replacement set up from Sonos at home is very worthwhile and with care in selection and placement, will most probably be successful.

What Sonos will NOT provide are the psychological underpinnings as to why the legacy hifi kit may still sound superior to some, including the eye candy like dancing backlit VU meters, glowing tubes and the like. For those that need that to complete the listening experience, Sonos isn't the answer, though a Sonos Connect can still bring many Sonos advantages without losing any from the legacy kit. Disadvantages of legacy kit like messy cables, large footprint, dedicated furniture to keep its components on, probable absence of instant start and the like will however remain.

More on the play 1: a linked review with measurements and conclusions on the speaker, including when two are used as a stereo pair.
http://stereos.about.com/od/Wireless/fl/Review-Sonos-Play1.htm
Now I understand the potential problem with all reviews on the net and these cannot be ruled out from this review either, but it certainly deserves as much credibility as do casually tossed comments, I suggest. And this one has a semblance of measured performance information to back up the conclusions. Note also that this does not address what the Sub does to address the observed limitations of the unit when it comes to delivering credible low bass, and this review also predates Trueplay.

Beyond this, the advantages in general of a satellite active speaker pair + Sub set up - not just those made by Sonos - are fairly well known.

I don't see what the place of purchase has to do with it
Quite right, it should not. But I know of audiophile thinking that runs on the lines that Amazon/Best Buy = mass market products = products that by definition not able to meet their standards for sound quality. And sub consciously, for the need to be exclusive.
A similar line of thinking to that which runs: high price = high end = higher fi that cheap Sonos kit. Home Audio in particular is rife with expensive rubbish. Note that this isn't the same as saying expensive audio kit = rubbish kit.