ATC SCM 40 vs. ATC SCM 50

If you go ATC then you really should consider their active speakers. The difference between passive and active crossover will make an audible difference in clarity and effortless sound.

Since you really liked the 40 then you will also like any of the larger ATC's as the voicing, design and timbre is almost all the same (probably unique in the speaker industry is the way that ATC sticks to a design for decades and that all ATC's, from the 20 on up, sound nearly identical). Larger models just increase slightly the clarity, dynamic range, max SPL and bass extension.

The ATC 50 and 100 are available as kits from Wilmslow audio in England. They use to offer the active versions as kits also but they are no longer in the catalog. I have been thinking about getting some built myself. You can buy the whole package but it would probably be more practical to get the cabinet made here. If you are handy or have a good woodworking shop nearby this might be the way to go.

Only the 50 and up is the real deal. Get the active 50SL if you can.

i have the 20's, i heard the 35's and 40's, and they lack a bit of extra details due to the change in tweeters, but still sound freakishly amazing. I believe that 50 and up will be the REAL DEAL. The details and clarity of the 20's plus an even more pronounced midrange, and a huge bass driver for amazing bass impact and superb dynamics!

AHHHH..ATC is a speaker for LIFE!

I prefer the passive. I won't miss the pleasure of trying different amps. :)