Long post, lots to say -- probably will fail.
I have always found a difference between the 50s and the 100s. The 100s are bigger, deeper, more of everything, weirdly whenever I hear tracks on 50s I feel the 100s sound slower and more paced. But that's just subjective ****. Pinch o salt required reflecting my prejudices that bigger speakers give me a bigger window on the world.
If I were building the speakers myself, I'd make some ATC 50's or 100 passives with Wilmslow kits.
If I were buying second hand, I'd try to get an active pair.
New?
Don't be silly, they are ridiculously expensive..
As y'all kno. I paid £1,600 for my s/h Black ATC 100As and then spent some time and effort on getting them up to SCM100ASL Pro status with new tweets and fancy veneers which involved ripping them apart. The leftovers are forming my little home project studio DIY actives. The cabinets are nothing special -- tore them open and they have no mystery to me, the amps are nothing special but they are designed for the speakers and are a "known load". The speakers are however IMO special. Old designs, sure! But do you see any of the Tannoy owners here complaining about the sounds they get?
Of course not. Good design works and is timeless. Holography I'll get onto later but that is not what I want in a studio monitor. I want to actually hear what was recorded (and unlike many here I know what I recorded!) I want to disassemble the sound while I am listening to it, tear it apart, work out how it is structured and how it works. Emotional attachment and Holography is for pussies.
I toyed with passive but felt it's easier to just get an active crossover than make a Passive Crossover and hamper yourself with a passive boutique 2KW stereo amp.
My DIYs are using three possibly dicked about with PA stereo amps and a s/h dbx driverack PA = change from £500. This way I use amps with less power requirements than Passive and still get adequate levels though, due to the active operation. That Passive ATC X/O means there's a fair bit of attenuation applied, ATC seems to 'need' powerful amps for the voltage swing, and not for the reactive loading!
My rationale is that by listening to good 1st gen sample sources in my studio (I am moving to 20-bit/96KHz samples that I make on my field recordings -- some are now B format as well) jacked into speakers with little in the way, I may actually hear a cleaner signal than the ATCs I already have in the living room with a source media passed through multitudes of compressors, limiters filters, served up on stampoed melted plastic and wrapped in static attractors.
I also hope if I nail the dbx active crossover for the space I am building, the amp load may be better balanced and reduce the distortion to a minimum for that topology.
However this is not gonna be a "my Disco Amp is better then a Naim"-type epiphany -- PA amps have a really mediocre PSRR but by inherently bandwidth-limiting the input to the amps with the active XO in the first place they'll probably perform pretty well. I think this is why the Amps in the ATC100s aren't that special but sound fine. As I'm using a digital XO I'll see if a brick wall above 25-30KHz on the treble might improve things. i.e i'll treat all three power amps as bandwidth-limited and that should help things a lot. There's some other trickery but the acoustics guy I'm using will determine that.
Of course it could all sound pants and I'll just have to bite the bullet and buy a ****y-but-well-reviewed Arye audiophile amp and go passive or something, but we shall see -- build starts next week.
PS just playing µZiq's Drum Light via Spotify. Awesome. Props! Plus I hear what you are saying WRT all that background data. I love all that texture that the track has. Like the very particular levelling clip that Moritz von Oswald always uses on Rhythm and Sound and on Basic Channel mixes. It sounds like grime until you hear is texture and shape and then you go "Ahhh! I Geddit"
PPS and before anyone else says it...
there.
Keep the thread on track! 
