Monitors that play huge

Nice choice. Did you order them in Mocha or Bordeaux? I assume that you are getting the stand4 also. They make a difference. If you need to 'tighten' up the sound you can fill them too.

That being said they will sound good out of the box then get very flat like someone disconnected the tweeters then go to sounding great again. With the original C1's it was between the 100 - 200 hr mark. I can't say regarding the sig's because when I got mine I ran the 24/7 but swapped out my Octave V70SE (tubes) for solid state before going back to the Octave. I have read from others the break in for the MKII/Sig is not as long.

Yes there is a good following for Dyn owners
http://www.avsforum.com/t/686371/dynaudio-owners-thread

C1 require more power,opens at louder volumes while the s1.4 is not so picky abaut other components

The C1 MKII/Signatures play great at all listening levels unlike the originals and don't seem to need as much power (rated the same). So in my opinion if you like to listen loud then the originals are fine but if you mostly listen at lower levels the MKII's are much better sounding. I have a sub with my C1 signatures but honestly it only really helps with recordings that have extremely low bass notes that the C1 can't physically reproduce.