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I was switching off the power amp for the mains, instead of connecting the soundcard output directly to the BFD
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Switching off the mains power amp is the correct way of doing it actually.
The LFE does indeed extend out to 120Hz in the Dolby spec, but my understanding is that the sound engineers roll that off themselves to integrate it more smoothly into the standard 80Hz LPF (low pass filter) crossover provided by all home processors.
The actual spec'd information when given to the sound engineer goes all the way to 120Hz and then is digitally cut off. This would be horrible to listen to if sent to a sub, so I guess it's their creativity to roll it off (and a smart move).
Since the 'small setting' redirected bass is normally rolled off from 80Hz (when that's the crossover you choose) at 24dB per octave, I understand that the LFE channel is given similar roll off characteristics.
At 24dB per octave, the amount of signal left at 120hZ is quite weak. Myself, I wouldn't be concerned about any differences in setup because of the LFE channel.
brucek