Trying for many years to have a list of my top five favorite loudspeakers regardless of price now has a very unexpected speaker to my list.
I have long hated low efficiency speakers and usually disliked three+ way loudspeakers. They always tend to sound all over the place and dynamically inept no matter what amp you put on them.
All my favorite speakers have to be musically approachable - I want to listen to all my music and not be either bored or irritated with fatiguing treble. I can live with a little less accuracy when I am rewarded with the ever elusive "musicality" which is a pain to try and convey with words.
The ATC SCM 150 (passive) is likely to be the most accurate speaker in my top 5 list which also makes it somewhat aloof compared to some of my other top 5 speakers. What the ATC does that every other Pro-Monitor speaker I have tried for extended periods is sound accurate without ripping the source discs apart so much that all you hear is disjointed presentation. The SCM 150 has immense control through the entire frequency spectrum and there is just enough of that touch of warmth to make you want to keep listening. Auditioning with 40 and 50 watt Tube amplifiers KT 88 and EL34 based integrated amps both easily drove the speaker to louder levels than I would normally use. KT88s tend to sound more solid State like so it sounds arguably thinner and more airy and open but less weighty and created that aloof sensation. The El34 amp was less hyper detailed and richer with more of foundation in the bass - I think a higher end 845 or 211 based amp will meet somewhere in the middle giving the SS "grip" while maintaining integrity to the emotion sought by musicians.
They're not inexpensive at about $20,000 but they're deceptively low efficient (or the tube specs are grossly under-valued)
Pictured here are the amplifiers the store sells. My recently purchased Line Magnetic tube CD player is at left. The big long black 845 Melody tube amps are beside the speakers - The 40 watt Line Magnetic amp had no trouble with the SCM 150 but if you really like more gusto the Melody's are pure class A 150 watt beasties
Add me as a fan of ATC in general. Whether it be their own passive/active speakers or the use of their woofers in other manufactures' speakers I've rarely had anything but good experiences with ATC. Unfortunately I have not laid ears on the 150's.
The funny thing looking at the ATC 150 is they remind me of the first pair of speakers I bought that were any good. And I still have them - the Wharfedale Vanguard based on the E-70. Well remind me in terms of looks. The Vanguard was completely different in most ways - mostly 10 ohms, horn tweeters and only a 10 inch woofer in a 70 litre cabinet (hence E-70). Still...