Why an external crossover for some models?
Moving the inductors and capacitors out of the cabinet removes magnetic, mechanical, and thermal interaction with the ribbon — and makes the network serviceable.
An external crossover moves the passive network — the inductors and capacitors — out of the speaker cabinet and into a separate chassis. This is done for four distinct reasons:
Magnetic isolation
Large air-core and steel-core inductors generate and are sensitive to stray magnetic fields. Inside the speaker they sit near the ribbon magnets and near the signal path; outside, that interaction is removed.
Mechanical isolation
Inductors and capacitors are microphonic to some degree — they vibrate in response to sound and can modulate the signal passing through them. An external box isolates them from the vibration of the cabinet and the bass panel, so the passive components do not mechanically colour the signal.
Thermal management
High-power passive components dissipate heat. An external chassis can be ventilated and kept away from the heat-sensitive ribbon, so the component values stay stable during long, loud listening sessions.
Serviceability and modularity
An external crossover can be upgraded, re-tuned, or serviced without opening the speaker.
This is why the Studio MK2, Auditorium MK2, Aria MK2, and Atrium System all use external passive (or, in the Atrium, fully active) crossovers.