ATC SCM40

I would add that I think your amp would be ok with it. I would recommend a tube based preamp, mine work great with my Schiit Saga and Freya+, ARC SP6B, and my Cary SLP 05.

I run the ATC SCM40 v2 passive using a SST Son of Ampzilla II amp with no issues. I think it’s a fabulous loudspeaker, made even more fantastic by the $2000 price drop. The use of the ATC-built tweeter made a vast improvement over the Scan Speak ferro-fluid tweeter used in the earlier SCM35, which was sensitive to amplifier choice (but I still drove it well with a 1975 GAS Amplifier).

This speaker, because of its sealed box design, is relatively impervious to major room issues and speaker placement. I get holographic sound with mine about 2 feet from the back wall. If you are worried about room loading, the smaller ATC speakers may be a better choice. I run ATC SCM7’s in my bedroom that sit on a dresser and work well. 
I want to emphasize that these are not hard to drive. I run the SCM7’s using an Adcom GFA535 II amp driven by a Schiit Saga (with a Sylvania 6SN7W tube) and the sound is glorious. The tweeters in this series are relatively forgiving. go for it! 

@jallan That’s pretty accurate overview ! I remember that GAS amp (I hate to say I’m that old) as at the time, they were excellent for the money. Ampzilla/son of was a great amp, more reliable than many others at the time. SAE was similarly reliable, but the GAS sounded better. Those were still the days of Phase Linear ($1 per watt) 400 amps!

People use that phrase "power hungry" and I get it, that phrase is often "situational" because I think this opinion comes from trying a low output amp (50W-75W?) that is often not as well designed as the higher power stand alone amps. Many of these demo experiments are with integrated amps or stand alone receiver company amps -not a high end design. The larger amps are usually ruled out due to cost, so usually not in play in a target system where people are searching for a bargain. Good example is a Crown D150- did not sound as good as the DC300. So sometimes it was companies just tried to make a cheaper amp that was lower power to fit the market, not just downscale the better higher power amp. (for background, it costs nearly the same to build a 75W or 150W amp vs a 300W one- the big costs are power supply, metal, meters, etc).

40s are optimized for low end extension vs efficiency, this is ATC’s preferred way of approaching loudspeaker design. Amps are cheap in this era so it’s not so difficult to get a larger 200W amp vs a 50W one. Much better place to invest $ is in the speaker with better low end and find a way to get a larger amp- even if not immediately. The big payoff is better low end AND much improved dynamics, which people don’t often think about until they are in their living room listening to "Fanfare for the Common Man" or Dark Side, or something that has some dynamics to it.  Dynamics plays such a significant role in realism.

Brad

Cannot go wrong here.  Ran the ATC 40 actives for some years with great results. They are world class speakers in either version and without being able to A/B them (passive vs active), I'm sure the actives would be more dynamic and immediate with attack.  That being said, my wife runs ATC 20 passives set on Sound Anchor stands with a monster Luxman 509X integrated and they are also scary good.

If you do go with the passives, heed the advice given regarding really good amplification and upstream source/cabling as these transducers will glaringly reveal all that you've done correctly AND incorrectly.

I always thought the appeal of ATC, besides the midrange driver that had its day, but would now probably among a group of quality mids, was that they were active and could be relied on to sound consistent compared to passive brands?