My $.02...
I have both the original Auditor and Auditor M right now and must say I'm a bit on the fence.
The M indeed loses some of that signature SF midrange magic, but does add a bit more detail (not WAY more as some claim), clarity, size, dynamics, extension, and has a really nice sweet and delicate top end. Not that the original is a slouch in any of these areas, but the M improves upon these things.
The original I would argue, is easier on the ears and more seductive -- it is a very unique middle ground between the old syrupy, super lush, rolled off SF house sound and a more modern "hifi", extended, neutral sound. For classical, intimate acoustic music, and raw vocals I think it is the winner.
For me, because I have very eclectic tastes in music that includes some electronica, hip-hop, and a lot of rock, I will probably go with the M. If I were more or less strictly classical and softer music of the like, I would probably stick with the original.
Of course, the amp you choose changes things. Another things for me is that I just settled on amp -- the VTL ST-150 -- which I like a lot for its rich, lush, emotive sound, especially with stringed instruments and vocals, but in compromise found to be a tad too dark. Well, that's in the shop right now so I haven't had a chance to hear it with the M, but my feeling is that the slightly brighter, hotter character of the M will balance out that bit of darkness.
I will say this: it has been said that the M is not as high quality physically as the original, and I think that may be true. The original stands are heavier and more solid with steel bases while the M stands are lighter and feel more plasticky -- disappointing for $1200! The finish looks a bit darker and richer with the original, but what stands out the most is material surrounding the drivers. The original has a nice, thick, elegant, deep grained leather (or at least looks just like real leather), while the M has a cheap, thin, dry leatherette. The ScanSpeak drivers also seem to be *physically* better quality than the M, though the M *might* sound better.
All in all I'd say if you're a longtime SF fan you'd probably like the original Auditors better than the M.