I use a pair of HGS-15s with an SMS-1 (precursor to DD15) balanced (XLR) lines from the pre/pro to the SMS-1 and then to the subs. The subs are located behind the main LR speakers, crossover is set at the pre/pro to 60 Hz (the crossover at the subs is set to off), and the level of the subs is set using white noise generated by the pre/pro to match the level of the other 7 speakers. Even experienced listeners are unaware of the subs, except, of course, when they do something you know the mains couldn't be doing like shaking the room when there is an explosion in a movie or providing that more felt than heard subtle vibration when a low pedal note is invoked on a pipe organ. Setting the subs to too high a level is major inhibitor to good integration; you really don't want to be aware of the sub as a sound source.
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