Marck,
Having migrated from the Summit-X to the CLX myself, I concur with most of the comments above. They are hugely articulate and detailed in their presentation. I can hear singers swallowing (amongst other bodily noises) between phrases on recordings that, with any other speakers I've ever heard, this level of detail is lost. User211 is right about the lowest frequency register though. For some music (depending on your tastes), you will likely want to augment with a sub. The Decent i is a solid choice... and with a room as large as yours you might actually consider two subs. ML also offers a specially designed "CLX Filter" that you can fit into a Depth i or Descent i and should help simplify blending... but sub placement, phase matching, etc is still critical to obtain the most transparent match.
Difference between original and ART are, I believe, limited to trim shape and an upgrade to stainless steel binging posts. I do not believe there is any difference at all wrt panel design or crossover implementation.
Also, as someone pointed out above, the CLX absolutely love quality power. I was running a pair of 80W tube monoblocks and while they sounded wonderful, it didn't take too long before I realized there was a headroom issue. I've since replaced them with a pair of ModWright 600W SS amps and couldn't be more impressed.
I'll also add that, without exception, all of the posts I've read herein from CLX owners seem to agree that once they migrated, they never looked back. There is something very special about the ability of these speakers to dig deep into the soundfield and present EVERYTHING to the listener... at times, to a fault. So the other thing to be aware of at least is that bad recordings will indeed sound plenty bad via CLX (perhaps moreso than with any other speaker you've heard). Whereas good recordings of music you love will simply blow you away.
Cheers,
-Todd
(p.s. also no more annoying turn-off thump with the CLX since there is no internal amp
)