Hi
My experience is limited regarding ATC loudspeaker system, with only the SCM50 30th Anniversary(demoed) and the SCM7(own these ones) that I've heard. The sound is different, I use a vastly different XO setup, cabinet and overall design. Really the only thing they share in common is the fact they have the same mid and bass drivers. But if you look at a popular driver such as the Seas Excel W22 8" bass driver, this is used in many commercial designs yet sounds different in each, its all down to implementation.
Just adding a notch filter in an XO can turn one speaker that sounds harsh or bloated into a completely different animal, and that's just one seemingly small change. With so many variables changed between my design and ATC's its easy to see why they'd sound dissimilar.
You ought to give the DIY thing a bash at somepoint. Start simple and you'll be surprised at what you can do and there's always plenty of help on hand when things go wrong. I'd also like to hear about Simons Manger/Eminance OB's, anymore work on these Simon?