hi @ei001h
There are some dealers in the north east with 50s, so you need to call the main Lone Mountain office in Las Vegas for more specific info or look up the website.
Now to answer your question, comparing 40 to 50, the big picture idea is ATC focuses on low distortion drivers across all models as bandwidth gets larger. Then ATC explores the entire active vs passive issue with passive and active versions of nearly all models (like the 40 you mention and the models below in price and size and models above).
Most ATC models sounds quite similar and are classed by dynamic range capability and resolution as well as bandwidth. This means you can select a model based on what’s best for the budget, room size, SPL requirements, low end requirements, dynamic requirements, etc. So ATC doesn’t do the "larger/more expensive is always better" thing like most companies do.
Active gains you more resolution, better imaging and lower total system cost (you get 3 amps in each speaker as part of the active package that are all big linear power supply and low distortion Class AB designs). This enables phase adjustments and other things internally that you cannot do in passive system- no matter how good the amp is in said passive system (even ATC passive). In active, colors of the amplifiers and sources shine through more than in passive system due to the lower distortion nature of the design and removal of the masking distortion left behind by a passive crossover and cabling. If you are a tube lover, I recommend you get a great tube preamp-it will definitely sound like a tube system. Active reveals more about cabling, phono stages, cartridges, CD players- everything in front of the amplifiers/speakers. All of these values are really choices for you to make and not all of these values appeal to everyone. For example, if you are in a 10x12 room, this extra bass may not be worth it (a 10x12 room cannot support much below 55Hz or so). The "clearer below 720" value might be worth it to you if you listen to a lot of orchestra or a lot of vocals or piano and could be more important to you and this would mean you don't need a larger speaker. If you are an acoustic guitar fan and like that type of music, spending money on getting lots of deep low end by going larger may not pay off for you. Then something like the 20 might be better, small , lower cost, but still supere resolution.
So if those values of more low end, clearer lower midrange and bass and better dynamics attract you, then investigating the larger ATC models like the 50 would be a good idea. If not, the 40 is a great speaker in both passive and active forms. The sound would definitely be very similar between them.
Brad
Lone Mountain
ATC Importer to the US