Hola...MMM I think that the Ethos is the one that has a passive woofer. Theos does not. This is what ML says about the bass section of the Theos: 8" (20.3cm) cast basket, high excursion, aluminum cone with extended throw drive assembly, non-resonance asymmetrical chamber format, down-firing bass reflex port. And for the Ethos says: 8" (20.3cm) cast basket, high excursion, aluminum cone with extended throw drive assembly, non-resonance asymmetrical chamber format. 8" (20.3cm) cast basket, high excursion, polypropylene cone passive radiator.
My liking and vote is for the Vantage, because it has a self 200W power amplifier to drive the woofer. To my ears, the bass if more robust and easy to adjust, due to the +-10dB adjusting knob at 35Hz for standing waves of the room. Also, the system power amplifier will drive only the panel section, not the woofer. Also a reason why has better sensitivity too. The panel is almost the same size and I think are equal. Haqppy listening.