NHT introduced the passive subwoofer (SW2) with an 80W external amp over 24 years ago.
I don't see why so many people consider it difficult. It's the same concept as passive speakers. You hook the sub output from the AVR to the amp, just like you hook the speaker output from the AVR to the amp. Then you connect the amp to the sub just like you connect the amp to the speakers.
But yeah, when I tell people about passive subs, they look at me like "Oh my gosh, what do I do?"