Seller used to mate them with 272 to very good effects.
Anyway cannot help but feeling those classics are the ones to get over any entry model up to 40A
They have all the same driver from the entry level to the pro-line.
The only difference is the current capacities but this not have any incidence on the sound.
that’s what they call the S version. But in domestic use the S version is useless.
The cabinet and the finition is nicer on the classic series.
With the 40 you win the fabulous ATC mid dome.
Personnaly i would go for the 40 eyes closed.
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The 40 is a great speaker but it’s very room dependent. I tried the passive SCM 40 is my modest sized music room (3.5m x 3.5m) and the bass was too overpowering. My SCM 19s work much better on my room. I expect the 20s would work too.
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Hi I would have said the same before listening to the scm20/Naim system… That dome midrange does miracles for sure bit surprisingly the top end refinement of the new tweeter of the 20 SL made a difference that cannot be ignored as far as I am concerned.
FWIW, last evening I spent a couple of hours listening to a variety of music through the ATC SCM40A and SCM50PSLT speakers and it was a real treat. They were being driven by a NDX2 w 555PS into a 252 Pre w SupercapDR. I did not investigate the interconnects or speaker wires being used.
Both systems were fantastic and if I had the funds I would absolutely pick up a pair of the the 40’s if I thought I could build the system around them to feed them.
Good luck with your decision and I’m sure you will be happy with either set of speakers you end up purchasing.
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The 40 and the 20 sl have the same tweeter except for the S version like explained up there.
The presence of a separate woofer add a weight to the sound. The 20 have less weight. Our brain traduce this like a more “clear” presentation but the only real difference is more detail at the end because the tweeter on the 40 doesnt have to do a part of the mid range like in the 20 or the 19 or the 11 and the 7.
In the two way design of ATC the mid is made by the tweeter and the mid/woofer.
In the three way the tweeter have less frequency to reproduce because he doesnt have to do the mid part.
Same for the woofer
In a demo our brain compare the difference only between two sound, and it take some time to asimilate some kind of sound signature. ATC is of of them, but when you tasted it, good luck to come back !
But at the end, for musical reproduction, the 40 is more faithfull to the reality versus the 20.
After this if you prefer the 19 is just a matter of taste but i personnaly find the 40 way more natural and dynamic.
Just for the story : when i was buying my speaker i compare the 19 and the 40. At first i prefered the presentation of the 19. But with time my ears and my brain came more “educated” and i chose the 40 and i never regret it at all. The 40 is an amazing piece, all the three driver are simply fabulous.
Hope my story will help you understand your taste to do.the right choice.
The 40 isnt room dependant. they are sealed cabinet, so easy to position.
Maybe the bass was just too much for your taste with your equipment ?
3,5 m is small btw.
I just received back my hi-line from reparation and i was using the grey I.C. durinf the wait time.
The bass is much more present with the Hiline !
So it is crazy like just a little modification can change the presentation !
I remember if i have a good memory you have a big nap 300 or a 250 ?
They are big power amp, they must push a lor of bass in your small room 
The 40s did sound great but some of my music collection is very bass heavy and the bass output of 40s were just too much wherever I positioned them in the room. I’ve had my SCM 19s for over a year now. They are sounding sublime driven by 552/300DR 
(& are a much better match for my room size compared to the 40s)
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Hi I understand perfectly your reasoning on which I basically agree. I do not have to choose between the two loudspeakers or convince anybody. I Just reported my own impressions after a listening session. The tweeter I saw on the 40a was different from the one on the 20sl and sounded different. Being the two systems different mine is not a definitive statement. It simply happened that I liked more the 20sl driven by Naim than the 40a in a Cambridge/Cyrus system. I hope I clarified my humble opinion… 
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Dont be intimidated by the lenght of my post, i was just sayin some fact about my favorite speaker brand and clarify a certain misconception of them and their driver sadly.
Maybe you talk about an old version of the atc tweeter ?
Maybe you misunderstand the goal of my post ?
Anyway, i dont want to be disrecpectfull at all and just love to write with people like you who have a passion for Naim.
But like i said in my previous post, your taste is indisputable. Like bongoman prefer the 19 in his room !
The 40 is " better" but the 19 suit more his taste and that is the only thing it count when you talk about a passion.
The SCM40’s use the SH25-76 HF unit whereas the SCM20’s use the Super SH25-76S,
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Sorry, was the ATC SCM50 the active or passive one ? If passive, what was the amp. driving it ?
Cheers.
Agree mojo i auditioned 19 and 20 in the same setup. The 20’s are in total differrent League. Compared to 40 they are more natural to me but less envolving with hard rock
The speaker used was the SCM50 ASLT, active which I believe was about $27,000 USD. The SCM40A was 13,999 USD. My typo on the model number above.
Thank you for the info 
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No misunderstanding at all, I can feel your passion for the 40’s and I agree they deserve It…
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I had the same experience in my modest room. The 40s need a decent sized room.
I preferred Harbeth 30.1 to ATC 19. All the goodness, if not more, of the ATC 40 mids and top end but with less bass energy. Bass accuracy and speed on 30.1 not as good as 19 or 40 but good for a ported design.
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Ah right, thank you. Yes the 40A are amazingly good speakers. For ANY PRICE let alone what they are selling for.
I do get the occasional urge to step up to the 50A, but by all accounts just save up some extra and go for the 100A instead is the way to go.
agree with both of you.
Surely 40A deserves its merits
Cannot have the best of both worlds here otherwise very few if none 50A and so on would ever sell
Anyway if you re in the middle of the bridge you ll be happy in any side. For sure the 40’s are really impressing especially with classical and rock music…james taylor will be perfect in any configuration