In the early days the only difference between the pro ATC speakers and the domestic ones was cabinet finish. Relatively recently the bigger domestic ones have added a floorstanding version as well and subsequently other cabinet differences. The mid dome was only available as one model for many years. A more sensitive version of it is a relatively recent addition.
I am not familiar with the 2-ways, but have heard most of the 3-ways over the years.
BTW for those that didn't know, the model number is the cabinet volume in litres.
I've changed my plans from investing in a set of ATC 2-ways (SCM19 or 20) for the following reasons:
1. My new room size really probably does need a 3-way (maybe even 3-way plus sub). As good as the ATC 2-ways are, I think I'd outgrow them quickly in my new living room.
2. Upgrade path: Right now, with the Devialet 400, I can bi-amp (passive or active). For 2-ways, that's the end of the line. But if I invest in 3 ways, I can move from bi-amp to eventual tri-amp (another Dev 200).
3. The midrange driver: As good as the SB75-150SL is purported to be, it's really the 3" midrange home that is at the heart of ATC's most popular pro monitors, which maps to certain mastering studios / producers that I actually own.