At 12,000rpm, a dual flute is probably the better option. The dw660 spins at a ridiculously fast 30,000 rpm no load. A single flute wouldn’t work as good unless you are cutting something very soft like most plastics. That explains the chatter you got with a single, your chipload was too high at 10mm/s at 12,000rpm.
There is definitely a harmonic sweet spot. When I do full radial passes, sometimes I hear this ringing, almost screeching sound. I wouldn’t call it chatter. Chatter has a lower pitch rumble. At one point, it goes silent and you can tell that the settings and everything were just spot on. All I can hear is the sound of the dewalt like it is cutting through nothing but air. I also think it has something to do with the thickness of your workpiece.
I am going to begin cutting some serious parts out of .25″ and 3/8″ 6061 t6511, will post my results.