very interested in King Zephyrs.what are they sound like??Hmmmm...you think the 6M has a small sound ? It is famous for it's wide, dark tone.....
But everybody has their own tastes.
Of the three...the Martin will have the most 'bite' to its tone...the sharpest edge, the most projecting high end.
The Conn would usually be the darkest....most low and midrange overtones of the three...
The Buescher would be in-between as far as tonal qualities. More lows than a Martin, not quite as dark as a Conn. More edginess and 'pop' than a Conn, but not so much as a Martin.
All three are great horns. You have all three available to you ? Or did you just pick these from research ?
Some others which may be of interest, if you want to expand your possibilities: Old Keilwerths or Kohlerts.... King Zephyrs.... Holton Ideal, 265's or Revelations... or Beaugnier-made horns (Noblet or Vito, France).
If not, then as I said, any of those 3 are superb. And they all have their signature sounds (that's what's so NICE about vintage makers...they all have their sounds and they don't sound like each other).
But if you purely want to go 'darker' than the 6M....you may have to go to a different maker.
maybe you are right...Seriously, I think you probably have a tenor sound in your head when you say "fat, dark". I think any decent tenor will most likely get you closer to what you want than chasing around after various vintage altos.
Which alto player has the kind of sound you are aiming at?