A perspective such as audiotroy's is worth considering. One like roxy54, presuming it is not based on actual comparison, is of marginal value.
Having reviewed several Legacy speakers and hearing several ATCs, I find the comparison by audiotroy to be fair.
I pursued the Wharfedale Opus 2-M2 Bookshelf Speaker to gain the benefit of a large, 3", soft dome midrange without the price associated with a newer PMC or ATC speaker with a similar midrange. Imo the true benefit of that sort of speaker is the fairly unique 3" midrange. Otherwise, I don't find them terribly impressive.
A glance at the specs is in order and one does not have to do a direct comparison to know a few things, given the specs. The ATC is truly a puny tower, at 38" tall. Imo, that is a non-starter for a speaker that is supposed to be impressive. Conversely, the Legacy is a full 10" taller, which places the mid/tweeter on a more elevated plane to the ears. Further, and this is a big difference, that what audiotroy noted as bigger and deeper bass is not marginally different, but hugely different. The ATC is, frankly, unimpressive at 48Hz -6dB. That's not even below 50Hz +/-3db! I do mean UNimpressive for a floor standing speaker costing $8K. I wouldn't even consider reviewing them with those specs. Conversely, the Legacy Signature is rated at 22Hz +/-2dB, and that is a WORLD of difference in experience! You would immediately sense it as a far more robust, powerful and immersive speaker.
The one advantage I see with the ATC is the capability of being tri-wired, and yes, that could give it the edge in midrange, maybe. But, that, too, can be countered by the owner of the Signature SE simply utilizing parallel/double speaker wiring to its biwire posts. Note also the Legacy has contour switches for both the bass and mid/treble to allow better integration with equipment.
In regard to the soft dome mid versus the ribbon mid, preferences will be split among listeners. That is a fairly common outcome. I understand the characteristic differences between them, as I hear in my room both the soft dome mid/twee like the ATC and the Legacy dual ribbon mid/tweet.
I have reviewed several Legacy products and own the Whisper DSW Clarity Edition speakers following review. I have provided justification why one could consider the Signature near the same $8K price as better value and perfomer simply based on specs and general design of each speaker. YMMV
