Where to hear ATC speakers

love your detailed requirements list and thoughts OP
the active filters do not sidestep the first order phase issue, they just move the steep slope filters and phase shift upstream
try to go listen to the Quattros, Jim Thiel and Vandersteen cut from same cloth. I own both. and as mentioned get time in front of ATC speakers ( yes the unobtrusive black no fuss models ) in das studio, they certainly have many virtues.
your reference back to Viola unamplified is rare, most people chase tail trying to correct for all the aberations in the chain, including microphones....enjoy the music !!!!!

Jim Thiel and Richard Vandersteen more than understand time and phase, i would say they pioneered it, certainly stuck with it as an ordinal design principle over the years, including very precise crossover matching to drivers in the anechoeic chamber. You can run your balanced lines to small monoblocks right behind the speakers. But yes the ATC actives are very good as is the power amp they use to get there...many studios run them in racks.
i would say GO LISTEN !!!

Apples and salmon IF you can’t design a first Order filter / driver system

 @rosami what NAIM gear are you running ? You might be very pleasantly suprised. I am running a NAIM streamer / amp w TREO CT, great sound. 

@ahofer. There is quite an active group of Thiel followers/owners here - yes your speakers have value certainly- you owe it to yourself to listen to the Vandersteen also IMO
have fun listening, enjoy the music, grace and peace on ya
jim

 @rosami Certainly a path to get your Thiel fixed and they are wonderful! I am sure your naim gear provide that jump factor
you have already violated the naim game keep it all in the family philosophy with heresy Thiel ....the filters for the Vandersteen are not going to get in the way IMO
have fun enjoy the music !!!
jim